Petitioner Truck Drivers Local Union No. 807, International Brotherhood of Teamsters, Chauffeurs, Warehousemen and Helpers of America (“Local 807”) seeks review of a decision and order of the National Labor Relations Board (“the Board”) dismissing an unfair labor practice complaint against intervenor Wells Fargo Armored Services Corporation (“Wells Fargo”). 270 N.L.R.B. No. 106 (May 18,1984).
At issue is the validity of the Board’s determination that under Section 9(b)(3) of the Labor Management Relations Act (“Act”), 29 U.S.C. § 159(b)(3), Wells Fargo was privileged, following the expiration of its collective bargaining agreement with Local 807, to withdraw voluntary recognition of Local 807 because of that union’s status as a mixed guard union.
We find that the Board’s interpretation of Section 9(b)(3) was reasonable. The petition for review of the Board’s decision is dismissed.
Background
Wells Fargo, an armored ear service incorporated in Delaware, transports and delivers money, securities and other valuables for banks and commercial customers. The company operates two facilities in the New York area which employ approximately 93 persons as guards on a full or part-time basis. From 1948 until June 18, 1979, the two New York facilities maintained a collective bargaining relationship with Local 820 of the Teamsters. Local 820’s membership and leadership consisted exclusively of guards employed in the armored car industry. Local 820 was nonetheless a nonqualified union under Section 9(b)(3) because although it did not admit any non-guards to membership, it was affiliated directly with the Teamsters, who do admit to membership employees other than guards. Such a union will hereinafter be referred to as a mixed guard union. The last collective bargaining agreement between Local 820 and Wells Fargo was effective through March 16, 1980.
On June 18, 1979, Local 807, a much larger Teamsters local that admitted both guards and nonguards to its membership, notified Wells Fargo that by directive of the Teamsters’ general executive board Local 820 had been merged into Local 807. Wells Fargo recognized the union pursuant to a successorship clause in the bargaining agreement with Local 820. Over the course of the remaining months of that agreement, Wells Fargo and Local 807 both proceeded in a manner consistent with full recognition.
As the expiration of the bargaining agreement drew near, Local 807 and Wells *7Fargo held numerous bargaining sessions in an effort to reach a new agreement. The existing agreement was twice extended to facilitate further negotiations. Wells Fargo, however, was unable to obtain certain economic and security-related concessions which it considered to be essential, and therefore made a final offer which was forthwith rejected by Local 807’s membership. A strike commenced on April 14, 1980. Several further negotiating sessions proved unsuccessful in breaking the impasse. There is no evidence that Wells Fargo bargained in bad faith.
On June 2, 1980, while the strike was still in progress, Wells Fargo sent a letter to Local 807 which stated that as of that date Wells Fargo was “withdrawing the voluntary recognition accorded Local 807 IBT as collective bargaining representative of the armored car guard employees” at the two New York area facilities. Wells Fargo gave no notice prior to this letter of its intention to withdraw recognition.
Local 807 and one of its members, Fred Gaputo, each subsequently filed unfair labor practice complaints against Wells Fargo charging violation of Section 8(a)(5) of the Act because of the refusal of Wells Fargo to bargain collectively with a representative of its employees. After the Board issued a consolidated complaint on the charges of Local 807 and Caputo, an administrative law judge (“AU”) held hearings on the matter and issued an opinion in which he concluded that Wells Fargo’s withdrawal of recognition was in derogation of its obligation under Section 8(a)(5) of the Act to bargain with the union. He reasoned that the voluntary recognition of Local 807 by Wells Fargo estopped it from withdrawing that recognition under the existing circumstances. The AU also found that Wells Fargo’s decision to withdraw recognition was based solely on economic considerations and that any concerns about potential conflicts of interest testified to at the hearing were mere afterthoughts. Finally, the AU found that although Section 9(b)(3) of the Act precluded certification of Local 807,' it did not bar the Board from issuing an order compelling the maintenance of a bargaining relationship voluntarily established.
The Board, by a vote of 3-1, reversed the AU and dismissed the consolidated complaint. It held that the congressional purpose in enacting Section 9(b)(3) was so overarching as to privilege Wells Fargo in the circumstances of this case to withdraw recognition from the union regardless of whether its present motivation was merely economic.
Discussion
This is a case of first impression involving the effect of a prior voluntary recognition on an employer's privilege under Section 9(b)(3) of the Act to refuse to recognize and bargain with a mixed guard union. We decide it within a well-established framework of case law and Board decisions on related issues.
At the outset we note the limited review of Board decisions provided for by Section 10(f) of the Act. Under this provision courts have generally accorded great respect to the expertise of the Board when its conclusions are rationally based. NLRB v. Yeshiva University, 444 U.S. 672, 100 S.Ct. 856, 63 L.Ed.2d 115 (1980). Indeed, the Supreme Court has made it clear that if the Board’s construction of a statute is reasonably defensible, it should not be rejected merely because a court might prefer another construction. Ford Motor Co. v. NLRB, 441 U.S. 488, 497, 99 S.Ct. 1842, 1849, 60 L.Ed.2d 420 (1979). The Board’s interpretation should be disturbed only if it is “ ‘fundamentally inconsistent with the structure of the Act’ and an attempt to usurp ‘major policy decisions properly made by Congress.’ ” Id. (quoting American Shipbuilding Co. v. NLRB, 380 U.S. 300, 318, 85 S.Ct. 955, 967, 13 L.Ed.2d 855 (1965)).
Section 9(b)(3) of the Act was passed by Congress in 1947 as part of the Taft-Hart-ley Act. The section provides, in relevant part, that
“[T]he Board shall not
*8(3) decide that any unit is appropriate for such purposes [collective bargaining] if it includes, together with other employees, any individual employed as a guard to enforce against employees and other persons rules to protect property of the employer or to protect the safety of persons on the employer’s premises; but no labor organization shall be certified as the representative of employees in a bargaining unit of guards if such organization admits to membership, or is affiliated directly or indirectly with an organization which admits to membership, employees other than guards.”
The section performs two functions. The first clause limits the organizational rights of guards — they must be in units segregated from nonguard employees. The second clause, the one at issue here, denies mixed guard unions the right to be certified as a representative of a unit of guards. See, e.g., International Harvester, 145 N.L. R.B. 1747 (1964); Schenley Distilleries, 77 N.L.R.B. 468 (1948). Local 807 argues that there is a distinction between the two clauses: on the one hand, a mixed guard “unit” is inappropriate for any purpose; on the other hand, a mixed guard “union” cannot be certified to represent a proper unit but is not in and of itself inappropriate for all purposes. We shall discuss this distinction later, but for now turn to the parameters of the provision governing mixed guard unions.
It is clear that in light of Section 9(b)(3) the Board cannot compel an employer to recognize in the first instance a mixed guard union as the bargaining agent for its unit of guards. NLRB v. White Superior Div., White Motor Corp., 404 F.2d 1100, 1103 (6th Cir.1968). See also Teamsters Local 344 v. NLRB, 568 F.2d 12 (7th Cir. 1978); City National Bank and Trust Co., 76 N.L.R.B. 213 (1948). It is equally clear, however, that under Section 9(b)(3) an employer may voluntarily recognize a mixed guard union as the bargaining agent for a unit of guards. White Motor Corp., 404 F.2d at 1103 (employer may, if it wishes, recognize such a union, but absent such consent employer may not be compelled, directly or indirectly, to recognize the union).
We turn to the legislative history of Section 9(b)(3) for guidance in determining whether the policy behind the section requires or precludes the issuance of the “bargaining order” sought by Local 807.
This legislative history, 93 Cong.Rec. 6444, reprinted in II NLRB, Legislative History of the Labor Management Relations Act, 1947, at 1541, 1581 (1948) (“Legislative History”) (statement of Sen. Taft), shows that the section was a direct reaction to the Supreme Court’s decisions in NLRB v. Jones & Laughlin Steel Corp., 331 U.S. 416, 67 S.Ct. 1274, 91 L.Ed. 1575 (1947), and NLRB v. E.C. Atkins & Co., 331 U.S. 398, 67 S.Ct. 1265, 91 L.Ed. 1563 (1947), which enforced Board orders requiring an employer to bargain regarding a unit of its plant guards with the same union that represented the employer’s production and maintenance employees. Each house of Congress drafted language as a response to the Supreme Court holding: the House bill excluded guards entirely from the protections of the Act; the Senate bill included them. The conference adopted the present language as a compromise so that guards could retain rights as employees under the Act. The conference chose the language, it said, because it found persuasive the exposition by the lower court in Jones & Laugh-lin, 154 F.2d 932 (6th Cir.1946) that guards who belong to a union representing plant employees would experience conflicting loyalties in the event of a strike because their obligation to the employer and the community would be incompatible with their obligation to the striking union.
The legislative history indicates as well that the conference was impressed by the dissenting views of Board Member Reynolds in such cases as Monsanto Chemical Co., 71 N.L.R.B. 11 (1946), wherein he argued that the Board has a duty to decline the use of its processes in order to avoid encouraging the creation of relationships which are incompatible with the Act and are inherently unsound labor practices. *9Legislative History, supra at 1541. Senator Taft further stated that “as to plant guards we provided that they could have the protection of the Wagner Act only if they had a union separate and apart from the union of the general employees.” Id. at 1544.
Senator Murray, speaking in opposition to the proposed legislation, stated that “the conferees have adopted a new rule with respect to ... guards ... denying them rights under present legislation.” Legislative History, supra at 1572. In analyzing the impact of the legislation, Senator Murray further stated that “[t]he restriction on Board discretion with respect to ... guards would require the Board wholly to ignore existing and satisfactory bargaining pat-terns____” Id. at 1581.
The Board and reviewing courts have consistently found that although in enacting Section 9(b)(3) Congress may have focused primarily on the particular situation in Jones & Laughlin, the broad language which it chose to use encompasses not merely divided loyalties at a company plant, but the potential for divided loyalty that arises whenever a guard is called upon to enforce the rules of his employer against any fellow union member. Teamsters Local 71 v. NLRB, 553 F.2d 1368, 1373 (D.C. Cir.1977).
It has been held uniformly in the last thirty years, for example, that armored car security personnel, like those employed by Wells Fargo, are guards within the meaning of Section 9(b)(3). In Teamsters Local 851 v. NLRB, 732 F.2d 43 (2d Cir.1984), the court found courier drivers to be guards and noted that the Board’s consistent interpretation of the statutory term to encompass more than plant guards comports with the statutory language and the legislative history. The potential for divided loyalty is present whether or not the same employer is involved. NLRB v. American Dist. Tel. Co., 205 F.2d 86 (3d Cir.1953); Armored Motor Services Co., Inc., 106 N.L.R.B. 1139 (1953) (potential for conflict of loyalty, though not so far-reaching as in plant situation, nevertheless present among armored car guards). Since 1953 the Board has consistently refused to overrule Armored Motor Services Co. See Teamsters Local 344 (Purolator Security, Inc.), 228 N.L.R.B. 1379 (1977), enforced, 568 F.2d 12 (7th Cir.1978); Teamsters Local 639 (Dunbar Armored Express, Inc.) 211 N.L.R.B. 687 (1974).
Local 807 and amici argue that even though armored car security personnel are guards for purposes of Section 9(b)(3), that section is merely a narrow exception to the general thrust of the Act and therefore should not be expanded to allow for the unilateral termination of consensual bargaining relationships. In their view, the potential conflict of loyalty with which Congress was concerned is, at least in the armored car industry, speculative at best. In any event, forced continuation of a relationship voluntarily initiated, they argue, is fundamentally different from forced recognition at the outset of a relationship. In their view an employer cannot have two bites at the apple; once it recognizes a mixed guard union, it must continue to bargain unless it has a good faith doubt of the union’s continued majority status. This is the standard applicable to a collective bargaining relationship with a certified union. To hold otherwise, they argue, would be inconsistent with the Act’s primary concern for stability in bargaining relationships.
The fact that Congress expressly precluded the Board from certifying a mixed guard union as the representative of a unit of guards, however, is certainly evidence that Congress disfavored such relationships.1 Moreover, it is reasonable to infer *10from the statutory language and the decisions under it that the preclusion of certification portends more than merely a simple check on the Board’s power to certify the results of an election. See, e.g., Teamsters Local 71 v. NLRB, 553 F.2d 1368, 1376 (D.C.Cir.1977), in which the court stated that the “agency reasonably could conclude that an organization falling within the limitations stated in Section 9(b)(3) should not be allowed to invoke the Board s processes. The courts have recognized that although members of a mixed guard union have certain rights under the Act, the policy of § 9(b)(3) dictates that such membership not bestow all the benefits normally associated with belonging to a labor organization.” White Motor Corp., 404 F.2d at 1104. See also NLRB v. Bel-Air Mart, Inc., 497 F.2d 322, 328 (4th Cir.1974). The proscription against certification of mixed guard unions therefore means that there is little sense in an employee remaining a member of such a union. A guard can adequately protect his rights, however, by joining a union separate from that of ot er emp oyees.
We are convinced that, based on the language and legislative history of Section 9(b)(3), the Board was warranted in interpreting the section as proscribing Board direction to an employer to bargain with a mixed guard union despite prior voluntary recognition of that union by the employer, There is sufficient support for the Board’s conclusion that in enacting the statute, Congress knowingly decreased the stability of bargaining relationships in order to further its objective of protecting employers from the potential for divided loyalty. In view of this, we find no reasoned basis for a distinction between initial certification and compulsory maintenance of a voluntary relationship. A voluntary grant of recognition cannot change the substance of Section 9(b)(3).
The difference in the language Congress chose in discussing mixed guard units and mixed guards unions does not change our analysis. An employer who voluntarily recognizes a mixed guard union may not discontinue the relationship during the contract period. Burns Int’l. Detective Agency, Inc., 134 N.L.R.B. 451 (1961). The fact that employers have the option to recognize a mixed guard union voluntarily, however, does not require that the option be forever binding once accepted. Mixed guard unions are appropriate only so long ag an j consents to re nize them. White Motor Corp., 404 F.2d at 1103. Cf. Amoco Oil Co. 221 N.L.R.B. 1104 (1975) (noting that emPloyer fXfeSsly consented to bargaining relationship).
Congress precluded certification of such unions because it realized that employers should not be forced to bargain with mixed guard unions. Employers may choose at the outset not to rely on the strictures that Congress enacted for their benefit, but the policy concerns inherent in the statute re-qUjre that employers have the right to rely Qn it at some later point Indeed, the record reflects that> regardless of We]]s Fargo’s present motivation for withdrawing recognition, the potential for divided loyalty is clearly present and likely heightened
Local 807 s argument that estoppel should aPP^ to Prelude Wells Fargo from withdrawing recognition is inapposite. The policy considerations behind the statute override an estoppel theory, for to recognize an estoppel would result in a situation inconsistent with the purpose of the statute. Cf. Teamsters Local 372 v. NLRB, 735 F.2d 969, 971 (6th Cir.1984). Reliance on such cases as NLRB v. A. Lasaponara & Sons, Inc., 541 F.2d 992 (2d Cir.1976), cert. denied, 430 U.S. 914, 97 S.Ct. 1325, 51 L.Ed.2d 592 (1977), is misplaced. That case appiies only to the situation in which a certifiable union is voluntarily recognized, rather than going to the expense of an election. We are dealing here with an uncertifiable union.
*11Finally, Local 807’s claim that Section 9(b)(3) as interpreted violates its First Amendment right to freedom of association, as well as that of its member guards, is without merit. Balancing the public interest to be served, as found by Congress, against the minimal infringement, the provision is constitutional. See Teamsters Local 344 v. NLRB, 568 F.2d 12, 20 (7th Cir.1977). Section 9(b)(3) does not abrogate rights — it simply makes them less attractive. Guards can join any union they wish. Any diminution of their rights is not imposed by the Board, but rather is freely self-imposed by guards when they select a mixed guard union as their representative.
Local 807 argues that even if the potential conflict of loyalty does exist, it could be eliminated by less drastic means, such as leaving the employer to negotiate for a broad no-strike clause. The Board, however, has refused to adopt this approach in a case of initial recognition, Teamsters Local 639 (Dunbar Armored Express, Inc.), 211 N.L.R.B. 687, 689 (1974), and now refuses to adopt it even where there has been prior bargaining. We decline to overturn its rulings.
The petition is dismissed.
. Section 8(a)(5) is silent as to whether a union must be certifiable in order to invoke its protections. A suggested amendment, providing that only currently recognized or certified unions could bring unfair labor practice complaints against an employer charging a refusal to bargain, Legislative History, supra at 321, was not adopted by the conference. Id. at 1538. The refusal to require certification as a standing prerequisite for bringing complaints under Section 8(a)(5) does not support Local 807’s arguments as to Section 9(b)(3). Congress’ refusal *10may mean that uncertified unions which may be eligible for certification have rights under Section 8(a)(5), but it does not necessarily follow that uncertifiable unions have the same rights.