Master cylinder replacment? - Diesel Forum

AK94 · 07-24-2008, 03:05 AM

My brake light has been coming on and I have been geting a faiding pedal. My vacume pump is making a "click" sound. I pulled the hose of the vacume? (fogive my ignorance, it's what the master cylinder is mounted on)
When I pulled the hose off I covered it with my hand and the "clicking" stoped and the brake light went off. It was recomended that I replace my master cylinder but I just wanted another opinion and maby an explination. I am driving a 94 f-250 IDI Turbo. thanks for your input. you guys know a lot about these trucks.

M.L.S.C. · 07-24-2008, 05:53 AM

First off, what do you mean by "fading pedal" - you gradually loose brake assist to it (you have to step on it progressively harder to get truck to stop), or does it try to slowly sink to the floor?

See if I get this straight - you pulled the hose off the vacuum booster, you plugged the hose with your finger, and funky sound disappeared and red "BRAKE" light went off? If so, then the problem ain't in your master cylinder, but the vacuum booster tis mounted on, and that the hose attaches to.

How do the brakes feel now, with the hose disconnected - with the truck off and parked somewhere step good on the pedal, it shouldn't fade at all, well maybe just a tiny bit, but that's it!

AK94 · 07-24-2008, 01:07 PM

The "fading pedal" is simply the pedal slowly fading to the floor. So far the truck has always stopped, I just have to press a little extra hard.

yes, I pulled the hose off the vacuum booster and I plugged that hose to make the "click" sound stop and the brake light went off. (the truck was running)

(still running) When the hose was disconnected, (capped or uncapped) the "fading pedal" goes away.
When the truck is off I never get a fading pedal no matter what.

does this still mean that it is the vacuum booster?
thanks for the input.

M.L.S.C. · 07-24-2008, 02:24 PM

Aight, so you definitely got a vacuum leak at the booster - whether tis the booster's internals worn out, or just that check-valve the hose plugs into, that you gotta check separately.

The pedal fade seems typical for these trucks, IIRC there was a TSB or two issued for that, booster-related for the most part. Could be worn master tho, if it has developed an internal leak it will fade.

Pete here posted the TSBs not too long ago (maybe like a month), so do a search on "low pedal" and you should find the thread.

AK94 · 07-25-2008, 01:58 AM

So I replaced the master cylinder and bled the breaks out well and I am still having issues. My brake light no longer stays on consistintly at idle but seems to be more random, no real patern to it. If I hold at about 2000rpm the light does not come on. After testing the brakes for about 15min my ABS light came on and is staying on.
The "click" sound that my vacuume pump was/is making does not "click" if I hold brake pressure and fade the pedal all the way to the floor. (at the floor no click)
So correct me if I am wrong, but the brakes should, when first applyed, catch quite sharply and then have very very little to no fade without applying eccessive pressure.
The way my brakes work: As I apply them they squish slightly and with slowly increasing force the brakes grab gently and as I press quite hard I stop. Then my pedal slowly fades to the floor with the pressure required to stay at a stop.
does this sound like a bad booster?

cdnsarguy · 07-26-2008, 01:40 AM

Do the vacuum test on my photolink...........

AK94 · 07-26-2008, 02:27 AM

So mayby I just misunderstand the mechanics of the vacuum pump and booster, but I dont understand how they would cause a fading pedal. Do these not assist the brake? So if they were leaky or faulty would that not make the bake harder to depress? not easier?
Do my symptoms sound like a leak in the brake line? I read one place that my RABS solenoid may be to blame, since I have ruled out the master cylinder by replacing it.
I just dont understand how the pump or booster could cause my problems. And an explination of this would be great. thanks again...

Plywood · 07-28-2008, 10:55 AM

My guess is the fade is there either way, but you only feel it when you have the strengh of the vacuum booster to help you foot.
I had this problem recently after I had a bleeder screw that was not seating all the way and causing a leak.
Any wheel cylinders or pistons leaking anywhere?

LMJD · 07-28-2008, 11:46 AM

Quote:

Then my pedal slowly fades to the floor with the pressure required to stay at a stop.
does this sound like a bad booster?

That's always been the classic symptom of a bad master cylinder. HOWEVER, the RABS valve (mounted on the frame, driver's side by the bellhousing) will show the exact same symptom. Years ago Raybestos, Borg Warner, or another major brake component mfgr. came out with a service bulletin regarding the valve's failure rate, failure symptoms, etc. I bought my F250 new in '91 and including the original, I'm on my 3rd one. I took one apart years ago and due to the wierdo design it only takes the slightest bit of crud to have fluid start leaking past the piston during a brake application.

M.L.S.C. · 07-28-2008, 06:20 PM

Yeah, my RABS is being retarded now too - I'm very very close to pulling it off completely, like a weekend away close, lol

jtate · 07-30-2008, 09:27 PM

Quote:

Originally Posted by cdnsarguy

Do the vacuum test on my photolink...........

i agree with cdnsarguy on this one. try this, with the engine not running press on the brake pedal a couple times and relieve the vacumn out of the booster. now after you have done this hold pressure on the brake pedal and start the truck. as soon as you start the truck the pedal should go down just a little, if it doesnt raise the rpms on the engine with your foot still on the pedal and see if it drops a little. the reason the light keeps going off when you raise the rpms is because at higher rpms the vacumn pump might be pulling more vacumn causing the light to go off.

AK94 · 07-31-2008, 01:49 AM

Well I think that I have finally found the problems. What was really confusing was that I had two seperate issues I was looking at and thought it was only one.
It seems that my brake light is caused by the booster. It must have a small leak some place that looses the vacuum if the RPM's are low, and that would explain why my light goes off when the vacuum pump is detached and the hose is capped.
Could this somehow cause the fading pedal as well? I thought if the booster was going out, the pedal would become harder, and not want to sink.
So, assuming that it is my vacuum booster is this a failure risk problem? I am going to be traveling soon with my truck and want to know if this is an at risk part or if this is part that will slowly fail.
thanks

M.L.S.C. · 07-31-2008, 08:20 AM

Try throwing a new check valve with grommet on it, tis the plastic 90-deg fitting that goes between the hose and the booster - sometimes those leak, while the booster itself is just fine.

jtate · 07-31-2008, 06:42 PM

have you verified that it isnt the vacumn pump thats bad and not the booster yet?

AK94 · 08-02-2008, 02:29 PM

HOW could it be anything else than my booster?
When I cap the check valve at the booster the brake light goes out which would mean that everything from that point back works properly, right? If I eliminate the booster from the vacume system (and cap the hose) the light goes out and pump sound fine. If I plug the hose into the booster my pump sounds funny and never builds enough presure to make the light go off.
The check valve at the booster is only used when the tuck is off right? To maintain the preasure? I guess if it leaked, while runing, I would loose vacume, but when I cap it my light goes out..... so it seems like i does not leak.