need some help on this one. i have a 92 w250 the break pedal falls preaty much to the floor befor it build any braking presure. i have replaced the master with brand new one no rebuild. i have bleed all the lines 3 or 4 times and still see no inprovement. the pads drums are all almost new. 1 new wheel cyl. 2 new calipers in the front. 1 thing i did notice was the abs vavle thats up in the rear frame rails was bypassed by some one befor me. with the truck off i can pump the breaks up and they feel real nice but as soon as i start it i lose all presure. any ideas i dont know what to replace next. please help

 

I am wondering with you being able to pump up the brake, without the truck running, if you dont have a bad rebuilt master cylinder?

You have said that the RABS is bypassed, hydrallic or electrical?

Did your brakes work fine before this problem?

Glenn

 

i dont know the master cylinder is brand new. the electrical was still hooked up. but the hydrallic was bypassed . i read some where on here to try un hooking the wires to that rabs and if your breaks get better then that will tell you that the rabs is bad. befor i did that i coundnt get any presure out of the peadle. so i disconned and plug in again the rabs and now i can build pressure with the trucks off. i have only had the truck for a few weeks and they have always been like this since i have had it. is there one of those abs things that controll the front breaks ??????

 

Are you losing fluid ??

It sounds like the vacuum-booster is sucking away the fluid, instead of it making pressure to the lines.

That would explain the being able to get good pedal with the engine not running, then losing it when the engine is running.

The only thing that can cause this is a bad master-cylinder.

 

i was having similar symptoms as you describe. i let my brakes gravity bleed for several hours and went through several gallons of brake fluid....end result....much better brakes. i also adjusted my rear brakes. my brakes actually lock up now...a whole new thing for this truck...usually it would just roll forward till it hit something, like the car in front of me

 

nope brand new master i know all the lines are good idk how much vacume is the pump spos to make i think my problem might be in one of theses lines
or mabey a probortioning valve. is there front abs in these

 

Rear ABS only. Look at my last post on your thread in the "Help" section.

 

I agree with wannadiesel. Check the pushrod adjustment. Remove the master. Put a small dab of grease on the pushrod end and reinstall the master. Then removed it and see if it is touching. Adjust it till its just not touching.
Good luck.

 

Have you been losing any fluid? Mine did that after I replaced mine, had a very small leak that I didnt find until I pumped it up many times, then it started showing up on the floor. Just a thought.

 

nope not lossing any fluid what started showing up one the floor

 

Brake fluid did...there was a brake line fitting that needed that extra 1/4 turn to keep from leaking. It took about 6 weeks for it to show up, but I found out the hard way in traffic, at a red light.

 

yea idk mabey thats the next steep is going threw and try to tighten all the lines