'92 f350 brake issue

[7.34ever]

ok so this truck is a woods truck that has bout 245k on it. the brake pedal slowly faded away like the master cylinder was goin out so i threw a new one on, adjusted the rears, bled the crap outa everything, and same thing... if ya hold consistent pressure on the pedal, it will fade to the floor slowly.

this truck has rear abs but it's a very simple system with no leaks and the fluid level doesn't change at all. there are no return lines where fluid could escape back to the reservoir and the m/c bench bled really good. anyone have an idea?

[CHUCK]

bad vaccum pump maybe?

[7.34ever]

thought of that but wouldn't the pedal still come up hard acting like it had manual brakes?

[CHUCK]

i dont know how much difference between 90/96 the vaccum system are. When mine went out, I'd have brakes then the pedal would either get extremly hard or hit the floor....

[7.34ever]

wierd. the only time i've experienced the vacuum pump goin out, the pedal was hard as hell the brakes automatically become manual. this thing has me stumped bigtime cause the brake system is so primitive. i think i'm gonna pull the rear wheels off and chack out all the parts and make sure somethin didn't come apart. good thing it's a spare rig huh!

[fastback11]

Ford had a service bulleten out that says to use the master cylinder from '96. The piston bore is a little bigger and the increased volume helps the pressure hold better. I performed this repair and it works great.

[7.34ever]

thanks fastback11 i'll check that out. i didn't get to pull the rears off this weekend but hopefully next week. i'll let you guys know what i find when i get to do it. thanks again

[Vik101]

Just had brakes done at a shop and after they were done the mechanic says to me 'youre good to go'. As I drive home the pedal does the same thing you're describing. I get to looking and find that the abs valve? on the frame under the driver side cab has a bleeder on it. So I think all of the calipers bled but the valve wasn't - and its at an angle to trap air. Have not got to drive the truck yet but I had real good pedal after bleeding everything including valve.
Give it a try.

[CHUCK]

just done the rear brakes on my 88 f350. Blead everything except the master cyl. in this order...LR,RR, RABS, LF, RF. The pedal feels good till I start the truck. Going down the road, seems to be excessive pedal travel, and once you hit the brakes, If you let off the pedal and hit it again...real hard pedal. I'm leanning toward the vaccum pump, but it'll have to wait after christmas....

[takif350]

You should do the 96 master and blead the valve in order pass rear drivers rear the valve pass front and than the drivers front just make sure you always have fluid in the master after each wheel