dcsimg

PDA

View Full Version : 95 Ford E350 brakes - Vacuum Booster


Blue Goose
October 19th, 2006, 08:06 AM
The brakes on my 95 Ford E350 failed. The pedal would go to the floor with minimal braking effect. There were no visible signs of leakage anywhere and the fluid level in the master cylinder reservoir remained full. I assumed a bad master cylinder. I bought a new m.c. and bench bled it and bled the system.
The pedal was firm when finished bleeding but as soon as I started the engine the pedal would again go to the floor.
I tried a couple of things; I could after shutting the engine off, pump the pedal so it was firm again. If I held my foot on the pedal and restarted the engine the pedal would immediately go to the floor. I then disconnected the vacuum line to the brake booster; the pedal stays firm and the vehicle in a test drive stops fine from highway speeds (of course with much leg force without the power assist).
Any ideas of what would be my next step to correct this problem?

dkerr
October 19th, 2006, 01:38 PM
The usual things you think of when the pedal goes to the floor is air the lines, or leaking lines, this can be caused by low master cyclinder fluid levels, rusted brake lines, etc.

But waht you discribed is a bit odd, assuming that you have already confirmed that no leaks appear at any brake cyclindar / brake calibrar, shoes / pads are in good condition, no obvious other problems, I would suspect the power vacumn booster at fault.

Try listening closely near the power vacumn booster to see if can hear any hissing / air leaking around there when the engine is running and breaks are being applied, may take another person to help out.

I am not a mechanic but that is what comes to mind based on what you mentioned, although I have never experienced those exact same faults, usually when I have seen failures they just stop power assisting.

Bear in mind that replacing parts just to narrow down the cause to the point of replacing un-needed parts can sometimes be more expensive than having a mechanic check it out.

mdshunk
October 19th, 2006, 03:01 PM
Does your truck have ABS? I'm thinking one of the ABS modulators is F'd up, and bypassing most or all the fluid, and when you turn the motor off, it no longer has 12V power to do that.

Blue Goose
October 20th, 2006, 08:02 AM
I was wondering the same thing so I tried a few scenerios; first I tried with the ignition on but engine not running and the pedal stayed firm, second I removed the ABS fuse and had engine running and experienced the same soft pedal. Thirdly I disconnected the connector to the ABS valve block and to the ABS pump motor and still no improvement.
But that still may not rule a malfunction in the ABS valve block where an increase in hyd pressure with the power assist may enable a bypass in the valve block where without the power assist it may not. I do not have a hydraulic schematic of the block so I cannot determine if such a situation could happen.
I guess I could take the valve block out of the loop and see if that makes a difference.

Blue Goose
October 23rd, 2006, 09:52 AM
I bypassed the ABS valve block using off the shelf brake lines and fittings, bled the whole system and the brakes work better than when the van was new. I think I will do a post mortem on the valve block to see what the problem could have been. Thanks for the imput guys.

mdshunk
October 23rd, 2006, 02:41 PM
I bypassed the ABS valve block using off the shelf brake lines and fittings, bled the whole system and the brakes work better than when the van was new. I think I will do a post mortem on the valve block to see what the problem could have been. Thanks for the imput guys.
That is very interesting. I guess we were onto something, then. I'm betting your ABS light on the dash is permanently on now, huh? :bawling: In any event, post what you find if you tear that ABS manifold apart. This is interesting stuff.