91 F250 5.8L Brake mystery

[oldtruckjoe]

I have a stumper for me on my hands. i hope one of you fine folks can help.

I have '91 F250HD 2WD, 5.8L.

Brakes build pressure and are rock hard when engine is off. With engine running, brake pedal is spongy and goes right to the floor. I beleive i have a vacuum leak issue, but before i invest tons of time, i thought I would check with this forum.

Here is what has been done over the last 2 years to the brakes:

• New Master cylinder
• new brake booster
• new front pads
• new rear cylinders
• new rear shoes
• Brakes bled (RR, LR, ABS, RF, LF)

most of this was done for other problems.

I did a quick vacuum check by spraying carb cleaner on each line, one at a time. i didnt cover the whole engine, but quite a bit. no surge in idle.

suggestions??

Thanks!

 

[Ridgerunner]

Welcome to FTF Joe. Someone will be along to help.

 

[flareside_thunder]

First, check for fluid leaks. I'd be willing to bet that the booster diaghram has a hole in it though.welcome to the site.

 

[Beach66Bum]

Did you bench bled the new master cylinder according to the instructions that were included with it? You said you checked for vacuum leaks, have you checked the brake fluid reservoir, if it's low, could be a sign of a leak. Check for a leak at the master cylinder to brake booster. Sure sounds like a failure with the master cylinder, but try re-bleeding the brakes again and see if the spongy feeling is gone. Keep us up to date, and welcome to FTF

 

[SuperCab]

First, check for fluid leaks. I'd be willing to bet that the booster diaghram has a hole in it though.welcome to the site.

This was my first thought when I read it.

And welcome to FTF

 

[oldtruckjoe]

update on brake mystery

thanks for all the suggestions.

No fluid leaks anywhere.

i bench bled the master cylinder as instructed when installed last year. (spongy brakes pre date change)

the booster is new as of last year and the spongy brakes predate that too.

today was the second time i bled the brakes. still have the issue.

how would i test the booster to check for hole?

how do you suggest testing if the booster check valve is good?

also, how would i know if there is sufficient vaccum at the booster?

thanks for the welcome to the site. i refer to it often, this time i didnt find a similar thread to use for inspiration.

 

[flareside_thunder]

thanks for all the suggestions.

No fluid leaks anywhere.

i bench bled the master cylinder as instructed when installed last year. (spongy brakes pre date change)

the booster is new as of last year and the spongy brakes predate that too.

today was the second time i bled the brakes. still have the issue.

how would i test the booster to check for hole?

how do you suggest testing if the booster check valve is good?

also, how would i know if there is sufficient vaccum at the booster?

thanks for the welcome to the site. i refer to it often, this time i didnt find a similar thread to use for inspiration.

Need a hand vacuum tester to draw vacuum on the booster. If it bleeds down, it's bad. The check valve is easy, use your mouth.

 

[Fellro]

If you had a vacuum leak, the brakes wouldn't go soft, they would be hard like when the engine is off.

Check the rear shoe adjusters, if they aren't up , you can get a soft pedal feel. The booster seems to do like it is supposed to, when you fire the engine, the pedal should drop just a little. The booster increases brake pressure through engine vacuum pulling on the diaphragm. Low vacuum, less pull, so I think that part is fine.

 

[David1357]

I'm thinking "Bingo"!

Check the rear shoe adjusters, if they aren't up , you can get a soft pedal feel. QUOTE]

I put my money on Fellro! Just had that problem and that fix on mine... First tighten them from the access window, then if not resolved, double check the assembly in the drum. If the accuator does not engage the knurled wheel, realign or replace.

 

[oldtruckjoe]

Ok, i have been super busy and have not had a chance to check the rear adjusters. Between my post and the suggestion from Fellro, i bled the brakes and put almost a whole bottle through the system.

Out of necessity, I drove the truck today and it is acting quite different. here is what occured:

- started out driving beautifully with normal brakes.

- after a 20 mile drive and 1/2 hour cool down, i started out again and the front calipers were not releasing. how do i know, it was obvious the drive train was having to work harder, and if i coasted, i immediately slowed to a stop. backing up/going forward a few times broke them loose, but it happened almost every time i used the brakes.

- next, my soft pedal returned.
- after about a 20 mile drive, i stopped, checked the reservoir (thinking i had a leak), still full.
- After a 1/2 hour stop, everthing went back to normal. good pedal, no sticking, but the antilock idiot light and the brake light were on . the brake light eventually went off. antilock still on.

thoughts?????

 

[flareside_thunder]

Bad front brake lines(the rubber ones) or bad ABS valve.

 

[dustybumpers]

Bad front rubber brake lines, and heat is causing the calipers not to release.

 

[Sidewinder4.9l]

I'm gonna toss my vote with Charlie's....Those nice lil rubber lines ($15 per side) will suck together and cause this very issue....

It's even more fun, when one side decides to do this...