79 4×4..No Brakes
Question:
Hopefully someone in here can help me…I’m getting close to the end of my rope…I’ve got a 79 GMC 4×4 3/4 ton with disc front/drum rear brakes…I lost nearly all pedal resistance and can’t seem to find out what’s causing it.. I had brakes fine until one morning I had to apply the brakes very firmly after a car pulled in front of me…the next time I went to apply the brakes, the pedal went almost to the floor, and in a panic I attempted to pump them up rapidly, but there was very little braking action..I was able to put the truck into reverse and avoid an accident… Here’s what I have done to try to fix it: Replaced master cylinder (twice)…each time I’ve bled it using the plug/hose kit that recirculates the fluid back into the reservoir Bled the entire system at each wheel (twice, using a over a quart of fluid) Manually adjusted the rear drum star wheels till they were tight Replaced the vacuum power booster Still, I have almost no pedal until it nearly hits the floor…I feel a springy resistance up to the point that they get firm, but the travel has nearly run out by then…I don’t know what else to try…Anyone have any ideas? Mike
Response:
Also, there are no fluid leaks…I’ve pumped the pedal 30 times or so and checked for leaks but found none….also the master cylinder reservoir was still full… "
Response:
Did you measure the drums for thickness? If they are worn beyond the thickness stated in the service manual, you’ll never get firm brake pedal. Other possibilities are old/contaminated fluid, improper bench bleeding of the master cylinder, improper bleeding of the lines, etc. You can check for a properly bled master cylinder by disconnecting the lines and plugging the ports. If the pedal is now firm, the problem lies elsewhere. Dean – Hide quoted text — Show quoted text – >Also, there are no fluid leaks…I’ve pumped the pedal 30 times or so and >checked for leaks but found none….also the master cylinder reservoir was >still full… >"
Response:
does this vehicle use a proportioning valve? because I just finished a nearly identical project on my Dodge B-350 van (84) finally replaced the P.V., all is well.
– Hide quoted text — Show quoted text -> Hopefully someone in here can help me…I’m getting close to the end of my > rope…I’ve got a 79 GMC 4×4 3/4 ton with disc front/drum rear brakes…I > lost nearly all pedal resistance and can’t seem to find out what’s causing > it.. > I had brakes fine until one morning I had to apply the brakes very firmly > after a car pulled in front of me…the next time I went to apply the > brakes, the pedal went almost to the floor, and in a panic I attempted to > pump them up rapidly, but there was very little braking action..I was able > to put the truck into reverse and avoid an accident… > Here’s what I have done to try to fix it: > Replaced master cylinder (twice)…each time I’ve bled it using the > plug/hose kit that recirculates the fluid back into the reservoir > Bled the entire system at each wheel (twice, using a over a quart of fluid) > Manually adjusted the rear drum star wheels till they were tight > Replaced the vacuum power booster > Still, I have almost no pedal until it nearly hits the floor…I feel a > springy resistance up to the point that they get firm, but the travel has > nearly run out by then…I don’t know what else to try…Anyone have any > ideas? > Mike
Response:
yes…it has a combination valve that, according to my understanding, does 2 things: it functions as a pressure differential switch, and if the pressure difference between the front and rear varies too much, it causes the brake warning light to come on…the other thing is that is supposed to delay application of the front brakes until the rear brake shoes have contacted the drum..at least that’s what a text book I read says…
– Hide quoted text — Show quoted text -> does this vehicle use a proportioning valve? > because I just finished a nearly identical project on my Dodge B-350 van > (84) > finally replaced the P.V., all is well. > Hopefully someone in here can help me…I’m getting close to the end of my > rope…I’ve got a 79 GMC 4×4 3/4 ton with disc front/drum rear brakes…I > lost nearly all pedal resistance and can’t seem to find out what’s causing > it.. > I had brakes fine until one morning I had to apply the brakes very firmly > after a car pulled in front of me…the next time I went to apply the > brakes, the pedal went almost to the floor, and in a panic I attempted to > pump them up rapidly, but there was very little braking action..I was able > to put the truck into reverse and avoid an accident… > Here’s what I have done to try to fix it: > Replaced master cylinder (twice)…each time I’ve bled it using the > plug/hose kit that recirculates the fluid back into the reservoir > Bled the entire system at each wheel (twice, using a over a quart of > fluid) > Manually adjusted the rear drum star wheels till they were tight > Replaced the vacuum power booster > Still, I have almost no pedal until it nearly hits the floor…I feel a > springy resistance up to the point that they get firm, but the travel has > nearly run out by then…I don’t know what else to try…Anyone have any > ideas? > Mike
Response:
The truck is fixed….it turned out to be the power booster… thanks for the reply… Mike
Response:
– Hide quoted text — Show quoted text -> Hopefully someone in here can help me…I’m getting close to the end > of my rope…I’ve got a 79 GMC 4×4 3/4 ton with disc front/drum rear > brakes…I lost nearly all pedal resistance and can’t seem to find out > what’s causing it.. > I had brakes fine until one morning I had to apply the brakes very > firmly after a car pulled in front of me…the next time I went to > apply the brakes, the pedal went almost to the floor, and in a panic I > attempted to pump them up rapidly, but there was very little braking > action..I was able to put the truck into reverse and avoid an > accident… > Here’s what I have done to try to fix it: > Replaced master cylinder (twice)…each time I’ve bled it using the > plug/hose kit that recirculates the fluid back into the reservoir > Bled the entire system at each wheel (twice, using a over a quart of > fluid)
Did you bleed the brakes starting with the wheel that is furthest from the master cylinder? IOW, RR, LR, RF, LF — RD (The Sandman) http://mywebpages.comcast.net/rdsandman Whether a gun is used to save a life or to illegally take one, the only difference is whose hands are in control of that gun. The gun itself is neutral.
