Please help on brake problem - Diesel Forum

michael92flatbed · 05-27-2007, 05:38 PM

I just purchased a 92 F350 2wd 7.3 with a flat bed. The truck has 294k on it and the master cylinder was dry. First of all I pulled the rear drums and the right rear wheel cylinder had been leaking so I replaced it first. The shoes and drums look pretty good, i replaced the drums and adjusted the shoes out to a slight drag on the drums. The problem is after I bled the truck 3 times the pedal still will not hold pressure. I bled RR,LR, RABS valve, RF, and LF, in that order. The pedal is better but you have to pump it about three times for it to get firm, but then it slowly goes to the floor while you have pressure on it. I dont know what to try first, a new master cylinder, booster or vacuum pump? Any help is appreciated.

luckys420 · 05-27-2007, 06:02 PM

this sounds like the common brake fade problem. your truck probably stops fine and the brakes hold just fine. if your search around on this site the problem is fixed with an "upraded" booster. it has something to do with the vacuum constantly pulling or something like that. people have replaced everything and still have this "problem". of course it could be something else. someone else will be able to give you more details. but i know it has been talked about on here.

good luck

94_turbo · 05-27-2007, 06:21 PM

mc dry--seals inside the cyl wont hold--replace the mc--then rebleed--after you do the bench bleed of the new mc----have had a few bench bleed give me fits before you could no longer pump the cyl in--and somehave pumped right up----who knows????

michael92flatbed · 05-27-2007, 07:19 PM

I was leaning towards the mc also. I have bench bled a few before so I know what you mean there. So you think my vacuum pump and booster are ok?

aceohearts · 05-27-2007, 09:36 PM

[ QUOTE ]
The pedal is better but you have to pump it about three times for it to get firm, but then it slowly goes to the floor while you have pressure on it.

[/ QUOTE ]

Thats a typical sign of a master cylinder going bad. One tip, after you bench bleed it and install it, while the bleeder hoses are still attached, have someone hold it almost all the way down (put a 2x4 under the pedal to be sure). Then while they are holding it down, install the brake lines. That way when they release the pedal it should pull any air from the lines back into the M/C where it can filter to the top of the resevoir. I did this when I changed the M/C on my wifes car, and didn't even have to bleed the brakes afterwards.

cdnsarguy · 05-27-2007, 09:49 PM

If your pushing on the pedal MORE than you would at a normal stop at a stop light........ It may sink slowly that is the nature of the beast so to speak......

You will have many persons tell you that your M/C is fubar, Mines been like that since day one Jan 93 and still doing it .....but it stops my truck on a dime and gives a nickel change.... DO NOT use excessive force after you are stopped....... simple as that.

If you want to put new M/C and Booster ETC and continue the money game for a few months go for it.......