Brake Failure

[RoverRideAlong]

Well I have just replaced my master cylinder/brake booster this evening due to loss of brake pedal. The original symptoms were:no pedal, it would go all the way to the floor, the brakes would then grab lightly once at the floor-the harder you pushed. So first I bled the brake lines, everything went according to planned, then fired er' up and...the pedal still went to the floor. So then I resolved with replacing the master cylinder/booster with a working used one. The swap was easy, went well everything was done and filled it up. Went around bled all 4 brake lines, seemed like this was the problem, great pedal feel and when I started it up went back to the floor. There is a little more stiffness in the pedal now and stops a little bit better but it is nowhere close to where it needs to be. I plan on bleeding all the brake lines again tomorrow, ran out of time tonight. But from there does anyone have any ideas?

 

[RoverRideAlong]

Bump come on someone shed some light

 

[p m]

ABS modulator bad?

 

[ptschram]

Air in master cylinder?

The ABS shouldn't be doing anything with the vehicle stationary-emphasis on shouldn't! Disconnect the wires to the modulator and try it again to rule out the modulator.

 

[Mike_Rupp]

Was the new master cylinder bench bled first? That would explain the somewhat soft feel of the pedal if it wasn't. If the pedal goes all the way to the floor, that points to a bad master cylinder or a leak somewhere. Are you losing fluid?

 

[p m]

Went around bled all 4 brake lines, seemed like this was the problem, great pedal feel and when I started it up went back to the floor.

I'd unplug the electrical connection to the modulator (I assume all the valves should be closed without power) and check this again.
But air in the m/c can cause this as well (not all the way to the floor, but very soft).

 

[elcalvomike]

Having the same issue. Before I spend $300 on a new MC just wondered if the modulator was the issue??!
Thanks

 

[barefoot]

unplug it and see.....

 

[elcalvomike]

touche....thanks.

 

[barefoot]

did you just call me a douche?
















































hope you get it fixed man.....

 

[elcalvomike]

no luck. unplugged the ABS modulator and same...brakes to the floor.
also I am not getting the ABS 'thump' when I put the key into pre-ignition. It was there before. Any ideas why?

 

[ReVoR.ca]

Pull the vacuum line from the Booster (cover the end with something so the truck doesn't sputter and die on you. I pulled mine off at the manifold and covered the hole @ the manifold with a few pieces of electrical tape) and start the truck. I had this issue over the summer and it was the only way I figured out my booster had gone bad. Truck running with vacuum to the booster, my pedal was to the floor...with the vacuum disconnected my pedal was rock solid right near the top (still couldn't stop to save my life however). I've also got all my ABS fuses pulled so my modulator isn't part of the equation.

If you've got air anywhere in the MC or the lines, pumping the brakes should bring the pedal up to some degree, atleast in my experience.

I doubt you've had 2 bad boosters in a row but stranger things have happened.

 

[elcalvomike]

Yea Ive had air and bad fluid in the lines before..Pumping brakes built some pressure back up. Not in this case.
Great suggestion...going to try that tonight. Thanks.

 

[elcalvomike]

This is worked nicely. Same result as you had. Breaks firm...ur um...stiff...you get the idea with the vaccum disconnected.

Do I have to replace the whole MC or just the Booster (servo?)
I have the RAV manual but not sure which part needs replacing.
Thanks again for a great tip.

Pull the vacuum line from the Booster (cover the end with something so the truck doesn't sputter and die on you. I pulled mine off at the manifold and covered the hole @ the manifold with a few pieces of electrical tape) and start the truck. I had this issue over the summer and it was the only way I figured out my booster had gone bad. Truck running with vacuum to the booster, my pedal was to the floor...with the vacuum disconnected my pedal was rock solid right near the top (still couldn't stop to save my life however). I've also got all my ABS fuses pulled so my modulator isn't part of the equation.

If you've got air anywhere in the MC or the lines, pumping the brakes should bring the pedal up to some degree, atleast in my experience.

I doubt you've had 2 bad boosters in a row but stranger things have happened.

 

[ReVoR.ca]

I only replaced my Booster (yup Servo, same part) and it fixed the problem. I had to remove the lines off the master to get everything out however so you're looking at another brake bleed once the job is done.

There is a little O-ring between the master and booster that I've been told is very important to have in the right spot when you put everything back together. Beyond that it's a matter of undoing 2 bolts on the firewall, the brake lines on the master, and the vacuum line, voila! Oh and be sure to re-attach your booster to the proper hole on your brake pedal...I put it in the wrong hole and couldn't figure out why it wouldn't make contact with my brake light switch (leaving my lights on all the time).

One thing to remember when you bleed your brakes, start at the rear driver's wheel...it's the furthest from the master because of how the lines are routed. I had to make a few rounds at all the calipers before my brakes were back to 'normal'...whatever that's supposed to be!

 

[elcalvomike]

Thanks for the clairificaion. Got a used part from Will Tillery at a great price.
Did you get a new seal or just use the one that was there?
I noticed a very small bit of fluid on the MC near one of the bolts.
It's not really leaking that much since the fluid level in the res. is fine.
Thinking the seal might also be shot. Thinking about replacing the brake lines with braided versions while I'm at it. Saves me another full bleed if I do it at a later date.

 

[ReVoR.ca]

I put in a used booster aswell, courtesy of Drillbit...works great!

I used the o-ring that was on it. It will probably stay attached to the master when you pull everything apart if you're more careful than I was...mine flew off and rolled beneath the truck. I didn't even know there was supposed to be an O-ring there until I happened to find it after dropping a wrench and came on here to figure out what the hell it was for.

I just remembered one other thing to watch out for when you take off the booster. The nuts on the inside of the truck...I nearly lost them (lost one of the washers completely, never to be seen again) behind the carpet and insulation...so don't be a butter fingers like yours truly!

 

[elcalvomike]

HA...I have two nuts a bolt, and one wrench stuck in the back door after replacing the vent window. I know the feeling. Thanks for all the great tips. Will update thread with my results.

 

[ReVoR.ca]

I have a buddy in OVLR with a wrench in the downpipe of his 109". I'm not sure how many years he's been driving it around like that either...

No problem, I'll check back to see how it went.

 

[elcalvomike]

Just an update....got used booster installed. And with out even bleeding the lines yet, brakes better than ever...Thanks again to Will T for getting me a sweet used one. And to ReVor.ca for the great tip to figure out it was a booster issue. Only thing is....brakes lights are in on position all the time. But I think somewhere in this or another thread this is discussed.