ahickman

94 4Runner, 3.0 4WD. Been round and round on the brakes. Bleed per FSM. Still can't lock up the wheels under any condition excepting gravel surface.

Tested the brake booster per FSM. Nothing terribly suspicisious except on the air tightness check. Depressed pedal with engine running. Shut engine off and felt the pedal rise forcibly. Continued to hold pedal down per FSM for 30 seconds. No significant change in pedal reserver.

Question would be is it normal for pedal to rise significantly when shutting off engine?

One last thing to mention. With the engine running, I hear a hissing sound from the booster area if I hold the brake pedal down. I would assume this is not normal (cracked booster diaphram)?

Really want to be sure my issue is the booster before investing in a replacement and want to get these brakes tip-top for safety.

Aaron

abecedarian

should not hear any noise when you hold the pedal down. that indicates a vacuum leak around the pushrod from the pedal

ahickman

Reason enough for not being able to lock up the wheels?

abecedarian

that would depend on if you have abs or not... and if the brakes have been blead thoroughly.
Also, if you have larger than stock tires that could make it more difficult to lock up the brakes under 'normal' circumstances.

ahickman

Rear ABS I do have. Brakes have been bled per FSM. Explain the ABS effect on booster action?

GSGALLANT

I think he meant that ABS should be preventing your wheels from locking up. At least the rear wheels, anyways.

For your booster problem, while the engine is running, even with a vacuum leak, the fact that there is continuous vacuum from the engine probably allows your brakes to work OK (pressure behind booster diaphragm will likely still be below atmospheric), but not as well as if you had the full differential pressure (normal engine vacuum to atmospheric).

When you shut down the engine with your foot on the brake, if there is a vacuum leak at that point, you will lose the "power assist" for your brakes (both sides of the diaphragm in the booster will go to atmospheric pressure, so it would be normal for your pedal to want to push back up against your foot (well... normal with a faulty booster, I mean).

ahickman

Thanks for the clarification. Just assumed that if the booster, tubing and check valve were all in good order, there would not be a significant loss in vacuum.

GSGALLANT

You can check to see if your system is holding vacuum by disconnecting the vacuum hose between the check valve and the brake booster an hour after you shut the truck down. If your system is tight, you should hear air rushing in when you disconnect the hose.

Then, if that checks out OK, do the same test you did before with your foot on the brake when you shut the engine down. When you feel the pedal force itself up, keeping your foot on the brake pedal, get somebody to remove that hose again. I bet there is no vacuum in the system at that point.

Although, the problem area at that point could be either a bad booster or a bad check valve, but judging by your description of the symptoms, I would put my money on the booster, because a check valve that doesn't hold tight when it's closed shouldn't affect the brake operation when the truck is running.

ahickman

Tight as a tick! Three hours later, I pulled the hose from the booster and significant hiss as air was sucked in. Safely assume the booster is good to go?

ahickman

Anyone?

GSGALLANT

Well, did you try this yet?

ahickman

Foot on the pedal, shut off engine and there is little to no vacuum on intake side of booster.

GSGALLANT

I'd suspect the booster at this point.

ahickman

Thanks.