I can't help with GMC numbers, but Chevrolet used a Bendix 378904 (Chevy 3757922) from 1958-66 and from 63-65 ALL w/ HD Brake Booster used Bendix 2501214 (Chevy 2445987).

I'm betting GMC used the same parts as Chevrolet...

Are you sure the pump you removed was pressure and not vacuum? Chevrolet had vacuum pumps on some trucks in the mid/late 60's...

Hope this helps,

Mike B smile

Mike Boteler

1956 Chevy 3100 Resto Rod
1952 Willys M38 Army Jeep
1983 Jeep CJ7 Laredo
1984 Jeep CJ7
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Silver Spring, MD