Spongy Brakes - need some advanced help.

[twinturbozs]

Im trying to diagnose my spongy brakes. Please read the facts before blurting out a recommendation.


1. 14" w/6pot Precision Calipers
2. 3 different mechanics have swapped 3 different Master cylinders
3. The pedal gets nice and stiff with engine off when pumped
4. When the engine starts the pedal get spongy
5. The car stops, I can lock up the brakes but the pedal never stiffens.
6. I gone through about 2 gallons of brake fluid, even tried pounding the calipers and ABS unit with mallet while bleeding.
7. The idle drops if i pump the brakes while engine is idling
8. There are no fluid leaks anywhere.

I undertstand a leak in the brake booster will cause the pedal to be stiff with less braking force.

Even with a stock master cylinder, there should be more pedal travel with the BBK but it should still be firm at some point right?

What else can be the problem?

 

[tookwik4u89]

even tried pounding the calipers and ABS unit with mallet

I would just keep trying this..

OR..

Check that you don't have any seized caliper pins, if they dont slide, you can get a real spongy feel. Pull all the wheels, watch what each corner does while someone pumps the brakes. I know you said big brakes but is it fronts or all around?

 

[twinturbozs]

I have BBK in front and just recently installed rear MKIV brakes, which made no difference at all.

 

[IJ.]

Possibly enough of a mismatch in piston size/area to be running out of volume from the master.

 

[Aspec]

Im trying to diagnose my spongy brakes. Please read the facts before blurting out a recommendation.


1. 14" w/6pot Precision Calipers
2. 3 different mechanics have swapped 3 different Master cylinders
3. The pedal gets nice and stiff with engine off when pumped
4. When the engine starts the pedal get spongy
5. The car stops, I can lock up the brakes but the pedal never stiffens.
6. I gone through about 2 gallons of brake fluid, even tried pounding the calipers and ABS unit with mallet while bleeding.
7. The idle drops if i pump the brakes while engine is idling
8. There are no fluid leaks anywhere.

I undertstand a leak in the brake booster will cause the pedal to be stiff with less braking force.

Even with a stock master cylinder, there should be more pedal travel with the BBK but it should still be firm at some point right?

What else can be the problem?

Idle will drop when brakes applied.

Possibly enough of a mismatch in piston size/area to be running out of volume from the master.

Agreed. Six piston calipers may be to much for the master.

Also, what is the condition of your brake hoses? If they are original they may be "Ballooning" under the added pressure assist provided by the brake booster when the veh is running.

With that much volume needed I would consider going to braided brake hoses all the way around. That may be enough to compensate for the lack of volume provided by the supra's small master cylinder.

 

[twinturbozs]

I already have braided hoses on all 4 corners.

When i upgraded the brakes on my 240, i noticed that the pedal travel increased but eventually the pedal will firm up, it just went down farther. I assume it should be the same case here.

 

[Aspec]

Do you have any pics of the front calipers installed?

 

[IJ.]

Time to do the math and crunch some numbers.
(well actually BEFORE buying the callipers was time)

 

[Aspec]

Also, You stated you have MK4 calipers on the rear. I have not researched this swap yet. Are the MK4 Rear calipers a ratchet type with integral e-brake lever or is the MK4 e-brake inside the rear rotor like on the MKIII?

If it is a ratchet type I may have an answer to your problem.

 

[drjonez]

yikes, lots of posting w/o knowing...

anyway, sounds like you've got trapped air somewhere....getting air out of the ABS actuator is a HUGE pain.

yeah, the stock master really isn't up to the task....but it shouldn't cause spongey issues. you may want to check the booster to see how long it will hold vac, a slight leak could contribute to your feeling....likewise perhaps the firewall is flexing too much.

 

[twinturbozs]

Drjonez, i know youve been around for awhile,

how does one go about getting air out of the ABS unit? Ive tried hitting it with a mallet numerous times while bleeding the calipers.

How long should the booster hold a vaccuum?

 

[IJ.]

Not enough volume per stroke of the master will equal air left over in any highpoint in the system.......

Try a vacuum bleeder.

If the one way valve is functioning the booster should hold vacuum as long as the booster isn't leaking, 2>3 pumps of the pedal should exhaust it.

 

[drjonez]

...how does one go about getting air out of the ABS unit? Ive tried hitting it with a mallet numerous times while bleeding the calipers...

best way i've found is to throw it in the garbage. :1zhelp:

 

[Nick M]

Under about 4-5 mph, ABS is inactive, and the valves are opened just like conventional brakes. This is why there is no special procedure listed in the manual for ABS equiped cars. When the car is on the lift, you should be going 0 mph.

There are two types of leaks, internal and external. They are different types of "spongy" feel. An internal leak in the system will cause the pedal to sink to the floor. You can stop, but the pedal will collapse. An external leak is obvious, air has gotten in during parts swaps and is not out yet. The pedal is simply soft, and the car will take longer to stop, the air is being compressed.