Any brake experts? I have questions...

[jlk]

I've owned half a dozen or so 1974 thru 1979 Chevy/GMC 4x4 pickups and none of them had good brakes. I put NEW not rebuilt master cylinders on them and they all act the same way.

When I'm driving and have to hit the brakes like in an emergency situation they never lock up. The pedal is hard, not spongy, but the brakes just do not work great. I've put brakes on lots of other rigs and they work great. I've been working on rigs for over 30 years, not that that makes me any kind of expert.

My daughters 1990 Toyota 4Runner has excellent brakes. If I have to really hit the brakes when I'm driving it, it stops right now. The Chevys always are slow to stop and never lock up.

What gives? I had Les Schwab (local tire store chain) put a NEW master cylinder on a 1979 K-20 and it still had weak brakes. The "pro" at Schwab said these trucks are just like that. Would a bigger bore master cylinder off a later model help?

I've noticed that sometimes when the engine isn't completely warmed up and the choke is still causing the engine to high idle that the brakes are more sensitive and sometimes the rears will lock and slide. New booster maybe? Goofy proportioning valve?

Thanks!