Question
-------------------------------------------
The text above is a follow-up to ...
-----Question-----
hey van, i have a 90 model chevy fullsize 2wd truck with rear wheel abs. suddenly my brakes went from awesome to nothing in a matter of about 2 days. brake petal would go down to the floor and parking brake light would come on. normally no big problem, just meant master cylinder needed replaced, been there before. so we changed it. still cannot get any petal after massive bleeding, changed master cylinder again, nothing, we do this about 4 times. so, moving on, we replaced the line coming from the rear brake port on the master cylinder to the porportion valve, also changed rear left brake line. still cannot get a petal. later on we changed the front left which did have a collapsed place in it. still no petal. im talking this petal goes down to about the same level of height as the gas foot before catching, then when it does the parking brake light comes on and petal goes down even more. if you pump the brakes lightly while slowing down it does build up some petal and the parking brake light goes out after you hear and feel a light bump coming from the brake petal. i have taken the truck to everyone around this town including chevy garage. no one can seem to come up with anything. chevy garage changed my drums out and said the isolation dump valve was bad, of course they just wanted to put a brand new one on there but i thought was too expensive, so brought it back home changed the front brake pads and put new calipers on it. still no better. also bypassed the isolation dump valve and still didnt get any better. any help would be greatly appreciated. im about tired of fooling with this problem, been goin of for about 2 months now.
thanks van,
nick
-----Answer-----
Make sure the pedal, when released, comes ALL the way back, allowing the master cylinder piston to relax all the way back.
It may be your power booster is not returning all the way.
Do you have vacuum booster, or Hydroboost power steering/brake assist?
Go browse this forum.
http://www.chevytalk.org/fusionbb/showtopic.php?tid/150309/
It is quite interesting. His problem has been going on for a long time also.
Van
hey van, the truck has a vacuum booster on it. i was told that you could tell if the booster was bad by letting the truck run at idle to build up pressure, then shut the motor off and pull the vacuum line off of the front of the booster and see if it makes the sound of pressure coming out. i tried this and yes you could hear a good deal of pressure coming out of the booster. if this theory is true, then yes the booster is still good. also, the brake petal does come all the back to normal resting position. it seems as if the brake lines have air in them, but no air comes out after you bleed them, or seems like a bad master cylinder. is it possible to have 3 or 4 bad ones in a row? like i said before, everytime we had to put on a master cylinder it took around the 3rd one before we could get any good brakes out of it. its just this time we cant. any more info would greatly be appreciated.
thanks again
nick
AnswerIt's possible to get a bad master cyl, but three would seem like it should be something else.
First of all, did you bench bleed the master, and get ALL the air out of it by tilting it in different directions as you bled it?
As for testing the booster, the test you did shows it holds vacuum. But to see if it assists, have the engine OFF. Pump the brake pedal several times to drain the vacuum off. Then apply moderate pressure to the brake pedal, and hold it.
While holding the pedal, start the engine. If the pedal goes down as the engine starts, the vacuum booster assisted, and is working.
Are your rear brakes adjusted up?
Van
