S 10 brake booster causing spongy pedal? (Page 1/3)
Rickady88GT OCT 07, 09:57 PM
My 88 has a good pedal feel with engine off, but start the engine and the pedal gets spongy. I know the booster takes out some petal feel, and that the pedal will go down a bit, but this seems extreme. So does the booster go bad like this?
fieroguru OCT 07, 10:17 PM
If this has developed over time, it likely is the rear brake calipers having too large of a gap between the pads and the rotors.

The parking brake self adjuster inside the caliper pistons probably are not working like they are supposed to.
If these do not adjust, then as the pads wear, the gap between the pads and rotors increases. This will give you a very soft pedal for the first portion of travel as the pads are pushed through the excess gap. The pedal firms up when the pads contact the rotor.
Rickady88GT OCT 07, 11:25 PM

quote
Originally posted by fieroguru:

If this has developed over time, it likely is the rear brake calipers having too large of a gap between the pads and the rotors.

The parking brake self adjuster inside the caliper pistons probably are not working like they are supposed to.
If these do not adjust, then as the pads wear, the gap between the pads and rotors increases. This will give you a very soft pedal for the first portion of travel as the pads are pushed through the excess gap. The pedal firms up when the pads contact the rotor.



Thanks, I actually have the rear wheels off and bleeding the brakes. I did not notice that much of a gap. The gap is so slight that you can barely see light through it. Same with the front. The pedal feels ok with the engine off, no vacuum to the booster. Start the engine and instantly the brake suck. (In my opinion) I am just not sure what is in-between the pedal and master, so not really sure if this booster is an issue?
Rickady88GT OCT 08, 12:02 AM
After bleeding the brakes, I feel like something is still not right. I think I will replace the master. It feels like the pedal creeps down as I hold pressure on it. No leaks anywhere, so if the pedal is creeping down then fluid must be pushing back past the cups.
theogre OCT 08, 12:41 AM
Spongy & Low pedal do not mean same.
See my Cave, Brake Service
Can have either or both at same time...

You took MC out to replace the booster now spongy pedal = air in the lines or MC itself.
See my Cave, Bleeding MC notes as to why you "bench bleed" them.

Low pedal often is rear brake problems. See my Cave, Rear Brakes

But Low pedal can be cause replacement booster w/ wrong adjustment on output rod that push the MC.
Need to find or make special tool to set clearance right or will have low pedal or worse problems w/ MC not fully return to rest position.
Output rod likely has clearance of some X 100th to 1000th of inch to make MC fully returns but not cause low pedal. tool(s) in FSM and other doc's to set the output rod is setup for this w/o publishing the tool design or clearance value.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

SemperFi18 OCT 08, 02:44 AM
Check your proportioning valve before you mess with the master. If you ever bled the brakes to quickly (quick loss of pressure) either front or back, that valve closes off that part of the brakes as a safety. Pull the sensor off the valve and you should see a gap that the pin from the sensor sets in. If it's moved front or back you can move it with a narrow pick.

Pulled my hair out for weeks with this issue, spongy pedal because you only have half your brakes. Once I checked that valve I fixed it in two minutes. Gravity bleed after that
Raydar OCT 08, 06:39 AM

quote
Originally posted by SemperFi18:

Check your proportioning valve before you mess with the master. If you ever bled the brakes to quickly (quick loss of pressure) either front or back, that valve closes off that part of the brakes as a safety. Pull the sensor off the valve and you should see a gap that the pin from the sensor sets in. If it's moved front or back you can move it with a narrow pick.




Not to discount your remedy. Obviously, it worked.
But if the valve is moved far enough to affect your brake system, shouldn't it be tripping the sensor and turning the brake light on?
Asking because I've been chasing a "fronts lock up long before the rears" situation for a long time. (Yeah... I know that's how they're supposed to work. But not to the extent I'm seeing, I don't think.)
My pedal feels fine. Brakes are freshly bled, with no real change.
The only difference is the front pads are supposedly OEM. Bought loaded NOS front calipers. The rears are... something else. But the fronts do make a lot more dust.

[This message has been edited by Raydar (edited 10-08-2021).]

theogre OCT 08, 09:51 AM

quote
Originally posted by Raydar:
Not to discount your remedy. Obviously, it worked.
But if the valve is moved far enough to affect your brake system, shouldn't it be tripping the sensor and turning the brake light on?
Asking because I've been chasing a "fronts lock up long before the rears" situation for a long time. (Yeah... I know that's how they're supposed to work. But not to the extent I'm seeing, I don't think.)
My pedal feels fine. Brakes are freshly bled, with no real change.
The only difference is the front pads are supposedly OEM. Bought loaded NOS front calipers. The rears are... something else. But the fronts do make a lot more dust.

Yes, when the balance bar moves the brake light should be On when the Key is On.

The bar isn't easy to move for bleeding and even many times driving w/ minor system leaks unless maybe you panic stop for whatever reason.

Balance bar Does Not close off a line w/ low pressure. In fact if the front has a leak etc will Open the rear to full pressure bypassing the prop valve.
If air got in the prop section and bar moves to front then may never get air out of that w/o bar reset automatic or manual. It doesn't block the prop valve but little or no flow when in bypass to push air out of the prop section.
Bar auto resets but often need to "slam" the pedal a couple to several times to spike system pressure until it centers and light off. If have air in the system maybe hard or impossible to auto reset depending how much air in there.

If balance bar blocks a line, then the combi-valve have other problems.

See my Cave, Combination Valve
mmeyer86gt/gtp OCT 08, 11:15 AM
I had this problem too. It turned out to be a brake bleeder split in a caliper causing air to get into the system.
Rickady88GT OCT 08, 11:56 AM
If the bar moves, shouldn't I be able to use an Ohm multimeter to check it, without removing it? It reads open right now, this is what I expect from a normal poison. If the bar moved shouldn't I get resistance?