About a week ago, I started intermittently experiencing the brake pedal dropping almost to the floor, then having it go back to firm. The booster also consistently makes a swish sound each time the brake pedal is depressed, but the swish is only when the pedal is traveling, it does not make a sound when the pedal is held down.

Brake system:
- Calipers - 3 months old, Napa rebuilt
- Wheel cylinders - 3 years old
- Master cylinder - 3 months old, A1 Cardone new, not rebuilt
- Hoses - 3 years old
- Dual diaphragm booster - scrap yard off a 1995 T100
- Booster check valve - 2 weeks, OEM
- Fluid - as clean and clear as it comes

First, I inspected for leaks and found none.

Second, I bled the brakes -- did not solve the problem. I had the pedal drop again within 5 minutes.

Third, I replaced the master cylinder with a new Aisin unit. Still did not solve the problem.

The problem has gotten worse...

With the motor off and the brakes bled, the pedal is firm and pumps up harder and harder. Once I start the motor, the pedal falls nearly to the floor and loses all firmness with the first depression of the pedal. Again, I have that swish noise each time the pedal is depressed.

I disconnected and plugged the booster vacuum line and drove around the block. I could barely get the brakes to lock up, but the pedal was firm.

It seems that I've isolated this to the booster, but the symptom is contrary to everything I've ever read... that when boosters fail the pedal just stays hard and you lose power assist.

Wanting a second opinion before I throw even more parts at this.

Thanks!

 

if you put your foot on the brake, then start the truck, pedal drop is normal, but probably not to the degree that you are describing.

on my truck, if it sits overnight, and you pull one end of the hose from the check valve to the booster, it's still holding vacuum.

 

I have changed a few boosters. They do go bad.

 

I know slight pedal drop is normal but this is a drop within an inch of the floor.

 

To me it sounds like the booster has a small internal leak, but I'm no expert.

 

My 86 has an issue with the booster I think. Does your idle drop and rise when you hold the brake?

 

No. My idle is stable. Yours sounds like the booster for sure.

 

Stanz,
If you do take it apart mind posting pics? I have a different intermittent issue with mine (86 Runner, manual tranny, 2RE) where pedal motion feels binding when I start to press, then releases after a split second. Stopping power is not affected.
I'd like to see what in the general assembly (pedal, booster, master cylinder) cuold cause it to bind.
CHEERS!
Ray

 

Argh!! The new master cylinder and booster are in, along with stainless steel hoses all around. Bled the $h*t out of the lines and it's still no better.

I pulled the drums and calipers, everything is clean and dry.

I can't win.

 

Unfortunately I won't be able to dissect the booster as I need to return it as my core

 

No worries. Thanks, Stanz.
I hope you get it figured out soon.

 

Since the pedal is firm with the engine off (relatively low brake line pressure), but sinks to the floor with the engine running (high brake line pressure because of the booster), it seems like you have a recirculating leak when the line pressure exceeds a certain value. I would guess that could either be in the ABS system (if your truck has rear ABS) or in the rear LSPV. I'm not familiar enough with either of these systems to know exactly how that would occur, but a leak anywhere else would result in fluid loss, which you make no mention of.

http://personal.utulsa.edu/~nathan-b...17brakeact.pdf
This section of the manual describes checking the rear ABS actuator. Note that the tests in paragraphs 1(c) and 1(d) describe your symptoms almost exactly, pointing to a defective/leaky ABS actuator.

 

Help! I know this is an old thread but I'm hoping the guy that started it might be able to tell me if he ever solved the problem? The description of the symptoms are nearly identical to what I have going on and I am at my wits end. Any help would be appreciated!

 

my issue ended up being mismatched wheel cylinders/calipers/master cylinder. It’s critical that the bores of all of these are in sync. Simplest method is to ensure they’re all matched to the same source vehicle.

 

Wyoming9 and the-supernerd - I want to thank you guys for your efforts and tenacity in helping me tackle this problem. I ran out of ideas on this a while ago. Thanks.

Yeah, these little motorhomes are overloaded and underpowered. That being said, however, they are also pretty amazing. We love ours. We lived in it full time for 3 years, put 40k miles on it, sold it, and missed it so much we bought it back 3 years later. Although right now I'm questioning my judgement on that decision.

One issue might be that I can't find the "proper" booster. There are none to be had for this truck that I can find. The closest I have been able to get would be to send the old one in to Rock Auto for rebuild since there are apparently no cores available. I found a link somewhere that said I could use a dual diaphragm booster from a 4wd 4Runner, so that's what's in there now. I also read that a mid-90s T100 booster might work - apparently there are a lot of options out there, and, if I can get this working, I'll be happy with the one I have. It's slightly larger in diameter and I had to fab a 3/8 spacer to get it to clear the steering shaft. I am certain the linkage and the actuator pin are adjusted correctly, and, when the brakes work, they work really well.

I like the idea of testing the pressure at the MC. I don't have a brake pressure gauge, but I may be able to beg, borrow, steal, or buy one around here somewhere.

The pedal is hard and high like one would expect from a well-bled system with the engine off. If I pump the brakes, hold the pedal down firmly and start the engine I get no drop, no boost, pedal stays hard. If I pump and hold lightly, the pedal drops a bit like one would expect from a good booster.

On the one hand, if it's the booster and I can get this put to bed I'll be a happy camper. OTOH, if it's the booster, I'm gonna be pretty pissed off that I've wasted all this time and money chasing down crappy rebuild parts. Sigh.

.