Soft brakes on 1990 GMC Sierra

I drive a 1990 GMC Sierra standard cab, 2 wheel drive, 5.7L (350ci), A/C, Automatic transmission, power steering, power brakes (front disc, rear drum), with just over 100,000 miles (170,000kms). No significant modifications have been done, other than transmission cooler, electric trailer brakes and Helwig overload springs. The truck has absolutely NO leaks and is driven gently and is well maintained.

The truck has been in my family for over ten years and has ALWAYS had a very soft and/or spongy brake pedal. The master cylinder and all wheel cylinders have been recently replaced. Brake lines have been professionally bled repeatedly with no improvement. Brake shoes and pads are new, rotors and drums have been machined and fall well within spec. Emergency brake works well and cable is new. Nothing at the wheels is frozen or seized or leaking.

Front brake lines are slightly cracked externally and need to be replaced but are NOT bulging, even under hard pedal pressure. Even when these lines were new, and with all new wheel and master cylinders, and after repeated bleeding, brake pedal was always soft. All that's left to replace is all the brake lines, and the proportioning valve (I'm not sure of the exact name, but it is located just below the master cylinder, has a bleeder valve on the top of it and has not been replaced while I have owned the truck).

I'm stumped (as are my mechanics and my mechanically-gifted friends). This has been driving me crazy for years, and would REALLY like to get it fixed.

 

If your brakes have ALWAYS felt that way and have worked fine for 100,000 miles I would say that that is the way the brakes are designed to feel! Compare it with another identical truck with the same brake system as stated on the Service Parts I D Label. I've gone the route with other folks with the same complaint and the bottom line is that that was the way the brakes felt on all identical trucks. Can't fix something if it isn't broken.

 

Are the brakes soft compared to other similar GM trucks you've driven or just seem softer than they should be - Compared to a Honda or something like that? Does the truck seem to stop ok, just with a soft peddle?

It sounds like you've checked and rechecked everything. Without driving it - the only thing I can say is GM is known to have very squishy brakes in their 90's model trucks. Both of the ones I've had were. You can help braking performance by manually adjusting the rears each oil change - the auto adjusters are notoriously lousy too. You can try stainless steel brake lines or performance break pads like Praise Dyno but I'm guessing it won't make a big difference.

Good luck!

 

Did the wheel cylinders leak BEFORE you say there was absolutely no leaks? Just curious because if they weren't leaking, there would have been no good reason to replace them, that I can think of, anyway.

And if the brake pedal did not pause, then continue on down to the floor, I would have seen no reason why the master cylinder would have been suspect either.

Because you had so much stuff replaced that contained brake fluid, you could have an air trap that never got out. Brakes are to be bled in a set order. I don't think everyone knows or thinks of this.

Have the rear shoes been adjuted so that they are softly up against the drum?

Perhaps there is a test where one can plug the rear brake line outlet at the master cylinder to see what happens to the feel of the brakes if that line and rear brakes were taken out of the equation, to see how the brakes would then feel. I have never tried this myself, but that might be worth running past someone in the know, like, well...maybe someone HERE knows...or someone who works at an auto parts store, or you could run it by a brake shop over the phone.

And as the posters before me stated, that naturally one would want to compare your vehicle to one just like yours, to see if that vehicle's brakes are as 'soft' as yours.

Now...by "soft"...describe what you mean exactly. Spongy? Are you able to differentiate between a brake pedal that travels for a bit before engaging?...and a brake pedal travel where there is fluid pressure being applied right from the get go, but for some reason is acting as if the pressure is being absorbed (like you said...like by rubber brake lines that could be expanding?)...or by air in the line? In other words...can you tell us if you are getting a free-play at all, first, or if the brakes feel like they are trying to brake with immediate pedal response, but that they are spongy from then on?