any auto mechanics here? 88 gmc truck brakes

ericlb · Dec 3, 2012

ive been looking for several hours on line and not finding any real answers for my problem, so ill take a shot here; heres the deal i recently bought a 1988 gmc 1500 extended cab 4x4 , 90,000 from new, the guy i bought it from just got too old to drive, the truck has had a easy life this weekend the brakes went soft on it, it was classic bad master cylinder symptoms, truck will stop sort of, once stopped, keeping pressure on the brake pedal sends it almost to the floor, been there done that a hundred times,in my life, on older stuff so, i put a new master cylinder on today, after making sure to bench bleed it until no air was present, i installed it on the truck and bled it a few times to make sure air didn't enter when i had the lines off, now i have the exact same problem i had to begin with! no lites on and the abs and brake lites do work with the key in the on position engine not running, i carefully checked over the entire truck, no leaks, no damage to any brake component, no air in the master cylinder, last week i drove this truck every day and it did not do this at all, any ideas here ? just cant see how something can do this and not see some kind of a clue, or how it can act like a bad master cylinder and not be that currently using my winch truck to go to work this gets 6 mpg! but its funny how these old dinosaurs are always in running condition when the newer stuff breaks down frequently

JayinNY · Dec 3, 2012

Just a suggestion, bad ,new master cylinder? Leaking rear wheel brake cylinder, or air still in the lines?

ericlb · Dec 3, 2012

it shouldnt be, when i bled the master cylinder on the bench, it was hard after the air got bled out,and wouldnt push in more than about 1/4 inch rear drums are dry and no sign of leakage

ray hofstetter · Dec 3, 2012

Is the brake fluid staying "up" in the master cylinder? I'd bet leaky seal on rear brakes, not always noticable right away....

ericlb · Dec 3, 2012

yes the fluid stays the same level after extended slow pumping i can see the fluid moving up from the ports in the piston bore, but no air, and the fluid level doesn't change

Fritz Maurer · Dec 3, 2012

If you didn't bleed the brakes at the wheels, you have an unknown situation that needs to be eliminated. There is no way to tell how much air did or did not enter the system when the master cyl. was off. Cracking the brake lines at the master cyl. will accomplish nothing.

to_many_irons · Dec 3, 2012

if the calipers are stuck on the pins can do the same thing .
so they are not sliding right .

ericlb · Dec 3, 2012

ok i will try bleeding at the wheels tomorrow, as well as pull the fronts and make sure the caliper isnt stuck it has to be something simple the thing drove just fine last week and nothing has happened to the truck

shaun wallace · Dec 3, 2012

Rear brakes may not be adjusted up. Either adjust up with brake tool or putting on parking brake and releasing several times. If adjusters not froze you wiil hear them click.

504 · Dec 3, 2012

As said check the rear brakes for adjustment, make sure the parking brake is not hanging up(if the pb is stuck, the brakes will not hold their adjustment). When bleeding the brakes engine off, and you will have to center the safety switch to get the light out.

toadady · Dec 3, 2012

before you waste anymore time! read [u:cec55c4b5a][b:cec55c4b5a][size=18:cec55c4b5a]THIS[/size:cec55c4b5a][/b:cec55c4b5a][/u:cec55c4b5a]

i have seen many wrench turners waste days because of this dump valve.

in fact you don't have to read the whole artical, just where it says PEDAL PROBLEMS

wilder11354 · Dec 3, 2012

Here ya go.
Untitled URL Link

ray hofstetter · Dec 3, 2012

Great info toadady, I've never got into ABS too deep - good
to know!

twopop · Dec 3, 2012

new brake booster

hoof print · Dec 3, 2012

Two words.....

Dump valve

MarkB_MI · Dec 4, 2012

There's a bleeder fitting on the unloader valve. Did you bleed it before bleeding the rest of the system?

Two Dogs · Dec 4, 2012

I went through the same with my 88 Silverado. It is the God-awful rear wheel antilock crap. I got a few fittings and bypassed that mess and now have good old fashioned brakes that work as they were meant to when we knew how to drive. You'll need to tape over the "check brakes" light to get any peace though. TDF

ASEguy · Dec 4, 2012

You need to isolate if it is an adjustment issue, air issue or mechanical issue. Pump the pedal about five times with the engine OFF. Let it sit for 2 minutes. Step on the pedal. It should be high and hard. Pull out the floormat and make sure your other foot will go under the pedal with the brake applied. If it's high and hard your adjustment is good enough if not it's either adjustment or mechanical issue, like caliper bushings; essentially anything which will cause too much movement of the caliper prior to brakes applying on the rotor. If the pedal is "soft" engine off it's either air or another hydraulic issue. Bleed all four if you can. No disrespect intended, but was the master cylinder bench bled prior to installation? If not, bleed all four brakes. Some air pockets can be a real pain to remove, so be patient, and if you don't have a helper, gravity bleed the system. If you still have a pedal drop you have a hydraulic issue. In that case , I would return the master cylinder as suggested as it's likely defective. Hope this helps. Gerard

ericlb · Dec 4, 2012

i think wilder hit on the issue that link sounds like what this thing is doing, im going to try to bleed that dump valve tonite when i get home, if that doesnt do it, i guess i need to take it to a real mechanic and let him fix it, i just hate to do that, actually i have a associates degree in auto mechanics but that was back in the early '70's and it sure doesnt do me much good now, the newer stuff [even though a 88 isnt new] just doesnt behave like i was learned that it should , like another poster mentioned if folks just knew how to drive we wouldnt need all this fancy crap , seems funny both my 80 and my 83 chevy 1 tons never seem to have any trouble, and my 94 dodge and now this 88 always seem to have some little deal wrong with them

GVSII · Dec 4, 2012

Had the same problem on a Jeep G/W. New M/C,bench bled but seemed to bench bleed too easyly and quickly. Installed ,same problem-soft pedal on every brake application. Second M/C and fixed.

LJD · Dec 4, 2012

Old body style Suburbans hold a lot of trapped air on top of the brake calipers. They HAVE to be bled at the calipers. I've seen many that when the calipers get stuck bleeders - attempts are made to bleed only the line connections, master cylinder, etc.

Just do the normal thing and bleed correctly.

If there is air in those calipers the brake pedal will sometimes catch on top and sometimes go all the way down.

Bryan inIowa · Dec 4, 2012

sometimes on head scratcher pedal issues I will block off the master cylinder . Remove lines , install threaded plugs into ports . If low pedal still then you know where the issue is .

ASEguy · Dec 5, 2012

Good point. I've done that on clutch master/slave cylinders.

any auto mechanics here? 88 gmc truck brakes

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