Brakes i have replaced every component in my brake system and still have spongy brake pedal. no air in lines. replaced booster, master cylinder, porportional valve, abs hydrolic valve, lines, calipers, and pads. brakes work but are spongy and when you apply them. some pressure is coming back up master cylinder and the pin in porportional valve is pushing out when brakes are applied.

This sounds like a vacuum or hydrolic issue. If The pedal is spongy soft while its running it indicates air in the brake line, or faulty master cylinder, or faulty brake booster. A faulty master cylinder would get hard after a few pumps of the pedal. Since the pedal is hard when it is shut down, it makes me lean toward the brake booster.

Hello Patricia,

Have you checked the master brake cylinder reservoir for brake fluid loss?

I don't suspect the master cylinder as you have replaced it... but am more suspect of a leaking brake line or fitting which should have
been discovered when you had the system bled.

The only time I ever had a rear brake problem with a 2500 series Chevrolet was due to seized rear brake caliper guide pins...as your calipers are also new that is not your problem.

If your brake pedal feels spongy and travels to the floor board you have air in the system, a brake fluid leak or a failure in the power steering system, such as a broken hose, broken power steering pump drive belt, or failed pump, would result in a loss of pressure to both the hydro-boost and steering.

Find more information here:

. Hydro Boost Power Assist Systems Operation Diagnosis and Repair

if you can see brake fluid around the booster then yes it is most definitely not right, but sometimes these can fail without any fluid noticable, the visible fluid could mean that the brake master cylinder is failing & this will have an adverse affect on the booster power & performance, check your brake master cylinder first as this is a much simpler & easier repair than the brake booster & can be done at home on a flat drive or or street even, a Haynes manual is invaluable for the home mechanic & i recommend you get one as soon as you can as this will help you diagnose the problems with your vehicle before going to a mechanic/garage, if it is the brake booster at fault then i suggest you go to your local scrapper/breakers yard armed with tools & the Haynes/Chilton's Manual (Depending on which side of the pond you are from) & remove the brake booster from a now deceased Toyota 4Runner & remove the brake booster & master cylinder in one should only cost you a few Quid or Bucks & replace the part once your vehicle is back at home, once the parts have been replaced the brake system will need to be bled so that there is no air in the brake lines, this can be easily done with two people at home, Jack up one corner, remove that wheel & then just crack off the bleed nipple of the brake line Putting a bit of clear hose over the nipple to catch the excess brake fluid in a jug etc & have a friend push down slowly on the pedal, when only fluid comes out with no air then the system is bled for that wheel, Check the level in the brake master cylinder & repeat the process for the other 3 corners/wheels Once done your brake pedal should fell firm & Not spongy, If you get a spongy feeling when braking after this then there is still air in the brake lines & the system needs to be bled again, of course if you have done it right then your brakes should feel firm underfoot. Please don't forget to rate me, regards, Paul.

air in the line or bad booster. try getting the brakes bled if it doesnt fix it probably a bad booster

check or adjust brake pedal, check brake booster or replace new one, check pedal return spring and bushing, check all wheel cylinder or replace, check brake shoe or brake pad, check brake fluid or replace,.

Is the master cylinder good? a bad master can cause this also did you bleed the master first before bleeding at the wheel and also make sure the bleeder screw is clean, if the hose is good then theres only a few things, a bad master or a master that wasnt bleed, try bleeding the master first then try rebleeding the brakes.

I would make sure all the air has been bled out of the abs unit and all other components. Any air in the brake system will cause a spongy pedal and is not acceptable.

ok the problem sounds like there is air in your system have you had bleed your brakes if that does not work have to get a new booster

leaky rear break cylinder or master cylinder

if the booster is bad the pedal would be hard sounds like the e-brake is stuck take off the drums and make sure the arms are all the way back then did you adjust the rear brakes after changing the shoes