1989 GMC 1500 4x4 Brake Problems

DocHEMI · February 2, 2014

Ok here's my truck....

1989 GMC K1500 5.7l rear abs system

Problem i am having is this.....

All new brakes (front and rear - pads, shoes, rotors, and drums), new master cylnder, new brake lines, new rear wheel cylinders.

Started doing all the brake work because old ones were all rusted and shot, just got the truck

Brake pedal had lots of trabel, but seemed to stop well, but only after going almost to the floor, so i started a revamp of the system, and figured a new master cyl wouldnt hurt.

Bench bled master and installed, bled entire system 3 times.

Pedal great and hardly any play with engine off.....start engine and pedal travels almost to the floor, brake work good tho.

Bled the entire system (pressure bled), still no improvement.

Only thing I can come up with at this point is the ABS hyd. unit under the master cylnder is no good.

Any ideas?

markw8 · February 2, 2014

I had an 89 and the abs module went bad. All that happened was the light came on and abs didn't work. Your problem is likely elsewhere.

Mark

DocHEMI · February 2, 2014

Well i was thinking that possibly the brake pressure was leaking by the accumulator and causing the problems.

I have no idea where else the problem could be

kstruckcountry · February 2, 2014

Can you lock up the brakes? If so, there's probably no "problem". Those old trucks make you push the pedal in a ways.

DocHEMI · February 2, 2014

Can you lock up the brakes? If so, there's probably no "problem". Those old trucks make you push the pedal in a ways.

only on gravel or lose surface can i get the brakes to approach lock up....but wait a minute....they shouldnt lock up (at least rears) with rear wheel ABS

Doug_Scott · February 2, 2014

Make sure you are bleeding the system in the correct "order". I do not have that info handy, but, you should be able to locate it online. For me to get it, I would have to search online, so, just as easy for you to look.

Even ABS will lockup on gravel for a short time, then release, then lock again and release. You should feel the pedal pulse when it does.

Instead of testing the brake pedal by pushing with all your might while truck is stationary and running, try driving it normally and see how the pedal feels. Also, if you have replaces pads, rotors, rear brake shoes and machined the drums you may get a soft pedal until the pads and shoes break in.

You can also try finding a way to apply the brake pedal overnight. I know it sounds weird, but, sometimes if you can get the brake pedal pushed down and held there over night the system will send trapped air back up to the master cylinder by the bubbles rising. Very common thing to do on motorcycles', and I have seen it work on cars many years ago as well. The bubbles appear to move up to the master cylinder, and once the pedal is released the brake fluid that gets pushed back into the master takes the bubbles with it.

kstruckcountry · February 2, 2014

You should be able to lock up all 4 pretty easily on gravel. I think the purpose of the ABS on back was to keep the rear end from stepping out. It never worked on my 1990, and honestly I didn't miss it.

gmtech4 · February 2, 2014

adjust rear brakes ,,,,that is the ONLY thing that would make the pedal low(without having the pedal pump up )(which means air )) ,,,,,make dam sure the bench bleed is perfect ,,,,,,this is not hard after all the parts that have been replaced

DocHEMI · February 2, 2014

adjust rear brakes ,,,,that is the ONLY thing that would make the pedal low(without having the pedal pump up )(which means air )) ,,,,,make dam sure the bench bleed is perfect ,,,,,,this is not hard after all the parts that have been replaced

OK...1. bench bleed was perfect

2. rear brakes are adjusted all the way out

3. bled all brakes again in proper sequence ...no air at all, no leaks, clean fluid thru whole system....Good pedal with engine off......star engine pedal goes to about 1" from floor

I am totally at a loss here

gmtech4 · February 2, 2014

what do you mean "all the way out"?

is it maxed out ?

DocHEMI · February 2, 2014

what do you mean "all the way out"?

is it maxed out ?

The brakes are adjusted out to meet the drums, actually had to back them off just a hair to get drums on, so they are adjusted Properly

Doug_Scott · February 2, 2014

The brakes are adjusted out to meet the drums, actually had to back them off just a hair to get drums on, so they are adjusted Properly

Did you back off the adjustment on the parking brake before adjusting the rear brakes? If parking brake is not adjusted properly you can't adjust the rear brakes properly. Back off the parking brake adjustment until the parking brake lever and strut bar inside the rear drums are loose and brake shoes are fully seated on the top anchor stud(don't recall the actual name of that part, but, it is where the long springs loop over).

I don't see how the brake pedal can be lower with the truck running, and be normal when truck is not running. The only difference between the two conditions is the brake booster not being active when truck is not running.

If you want to try something, block off or unplug the vacuum line to the booster and take the truck for a drive. WARNING: the brake pedal will be a lot harder to depress, but, it will depress and the brakes will work, but, it will be a lot harder to stop. I suggest you find a parking lot that is empty, then pull the vacuum line off and take the truck for a slow drive in the empty parking lot. I think you will find the pedal is still just as low though when you actually try to apply the brakes for real. It may take two feet to press the pedal hard enough to stop. I am only suggesting this so we can see if the pedal is actually at a norml height with booster disconnected from vacuum.