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!

[john@thepier]

My 85 K30 has hydro-boost and it will throw you through the windshield if you want it to. All my 97-2000 with hydro- boost trucks were like you just described. Difference was the anti lock braking they put into these trucks. I once had a 97 dully, brand new and if you came up to a stop sign/light aggressively, like I always do, and there was a slight bump in the pavement in the stretch you had your foot in the brake, it would go into anti-lock and you prayed no one was in front of you, well one time there was and I ended up rear ending the lady. Of course I always tell myself since then, to slowdown ahead of stops. I have always felt like you do, if I jam my foot down, brakes should lock-up. I'm sure the safety is well designed by the expert for anti lock though. Something you could try would be an adjustable proportioning block, and or braided steel brake hoses. Maybe one of the fellas here can tell you how to get more vacuum to your booster as well.

[Keith Seymore]

Originally Posted by john@thepier

My 85 K30 has hydro-boost and it will throw you through the windshield if you want it to. All my 97-2000 with hydro- boost trucks were like you just described. Difference was the anti lock braking they put into these trucks. I once had a 97 dully, brand new and if you came up to a stop sign/light aggressively, like I always do, and there was a slight bump in the pavement in the stretch you had your foot in the brake, it would go into anti-lock and you prayed no one was in front of you, well one time there was and I ended up rear ending the lady. Of course I always tell myself since then, to slowdown ahead of stops. I have always felt like you do, if I jam my foot down, brakes should lock-up. I'm sure the safety is well designed by the expert for anti lock though.

Keep in mind that ABS was developed to help with directional control during brake stops on low coefficient surfaces....not to improve stopping distance.

A trained driver will almost always be able to beat the ABS for stopping distance.

K

[argonaut]

Low vacuum signal to the booster will cause what you describe. Also a smaller diameter master cylinder will improve mechanical advantage. Hydrobooster will also solve the problem.

[61K10]

also the bigger the tire is, the harder it is to stop rotating.

[Keith Seymore]

Originally Posted by 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.

Just to make sure I understand the complaint: when you make these emergency stops is it (a) a hard, linear application of the brake pedal, or (b) a really rapid whack of your foot on the brake pedal?

If it is the latter, then the condition you are referring to is called "dashpotting" and is a characteristic of the vacuum booster. There were several engineering projects done over the years but none of them were successful in completely eliminating that effect. You won't be able to get rid of that without going to hydroboost.

K

[jlk]

[QUOTE=Keith Seymore;6987959]Just to make sure I understand the complaint: when you make these emergency stops is it (a) a hard, linear application of the brake pedal, or (b) a really rapid whack of your foot on the brake pedal?














(b) A really rapid whack.

[Keith Seymore]

Roger that.

No fix other than going to hydroboost (or manual brakes).

K

PS: 5000th post!

[RedRoad]

sorry to hijack but I have a HB setup in a C30 that works great but I feel a pulsing in the pedal when applied, is the pump or the box the culprit?

[57NAPCO]

Originally Posted by RedRoad

sorry to hijack but I have a HB setup in a C30 that works great but I feel a pulsing in the pedal when applied, is the pump or the box the culprit?

Most likely warped rotors...

[RedRoad]

It pulses standing still

[SeventyOne]

Sounds like the pump.

Other than hydroboost you can also do the 1 ton booster swap. Do a search there's a nice write up with pics.

[OrrieG]

My experience. I have a 74 drive train under my 59. 74 K20 MC, 8" hot rod booster, 74 proportioning valve. During my two trial runs when I hit the brakes it would stop right now and toss me thru the windshield. 18# vacuum. Before this build I ran the same MC without the booster for 15 years and never had a situation, panic stop on asphalt or holding, long down slopes off road, when the brakes seemed to not work.

[Bigdav160]

Before this build I ran the same MC without the booster

As mentioned above, a master cylinder meant for manual brakes (smaller diameter bore) is going to multiply the pedal force more than a larger diameter power brake master cylinder.

Back to the OP's complaint, I don't think it's fair comparing the brakes off a Taco to his truck. The Taco brakes were over built with fixed multi piston calipers and oversized rotors. People swap those brakes onto other vehicles including Z car nissans

[Mike C]

That '85 K30's brakes aren't anything like the K20's.

I have K2500 Suburban brakes on my Jimmy from an '86, and even with 35" tires it stops great. I have the original power booster from the half ton brakes and a half ton master.