Brake problem I replaced the booster two years ago. I just replaced the master cylinder. I bled it as instructed and bled all the wheel lines. I have a good pedal until I start the engine. It must be a vacuum problem. Where do I start???

Did you properly bench bleed the master cylinder itself before installing? It must be bled or there will be air trapped inside the master cylinder itself that no amount of wheel bleeding will remove.

Check rear of Master Cylinder--If wet likely Master Cylinder Leaking and Booster Damaged. They Should be replaced as a mated pair. (for warranty) and brake lines removed and system bled.

I've had the same problem before, ended up being a bad master cylinder that I just purchased. Replaced it with a different brand and it worked fine.

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.

did you bleed all of the air from the master cylinder before you installed it, this has to be done. also when replacing the master cylinder, all 4 wheel cylinders will have to be bled, in order to get all of the air out of them. while doing the wheel cylinder bleeding , you will have to make sure that the master cylinder doe's not run out of brake fluid, check it each time you bleed a wheel cylinder , add brake fluid to the master cylinder each time or when it is getting low. hope that this has helped you.

did you try a brake booster ?

Yes master cylinder and maybe ABS sending unit. Also is there a sound of an air leak it may also be your brake booster

Are you losing any Brake Fluid? Is it visible anywhere at the four wheels? Is it visible anywhere else?
The Master Cylinder may be defective. When you bled the Brakes did you maintain proper Brake Fluid Level in the Master Cylinder? If you didn't: and air entered into the Master Cylinder there maybe air within in the system in the Master Cylinder. You can bleed the Master Cylinder by removing the brake lines, adding more fluid, pumping the brakes, bleeding the air out of the system.
Are you losing Brake Fluid and can't find a noticeable leak? Then there is a good posibility that the rear seal in the Master Cylinder is leaking and the Fluid is going into the Brake Booster - which is the large wheel shaped drum located on the fire wall. In this case both the Brake Booster and the Master Cylinder will have to be removed and replaced. The Booster contains seals that Brake Fluid is very caustic to and will ruin these seals- causing future failures.
If no Brake Fluid is lost, no leak found: Check the Brake Booster Vacuum Advance. This should be connected to the outside of the Booster, attached to a hose, with the hose going to the engine. Check to see if this is working properly. Replace if needed.

5 Inspect Brake Fluid Type Brake fluid loss or contamination 10 Inspect Master Cylinder (Brake System) Worn or leaking brake master cylinder. 11 Inspect Brake Power Booster Damaged booster vacuum line/check valve, or diaphram