Thread: Need some help with the brakes

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I replaced my master cylinder and booster with chrome ones. I bench bled the M/C. When I run the engine, (no foot on the brake pedal) the booster pulls the brake pedal down and locks up the calipers on the front brakes. Shutting down the engine will let the pedal return to normal after a minute or so. One of my neighbors, who is not really a brake guy, says my booster is bad. Ideas??? Thanks!

That's really strange. Wish I had an answer for you. Maybe if you call the people who made the setup?

Don

Sounds like a bad booster to me too...not much else it can be really.

try backing the nuts off that hold the m/c to the booster till the last couple of threads pull the m/c back to the nuts start the eng. if it is ok the m/c rod is too long that happened to ME along time ago .....ted

Yep, the vacuum orifice is in the wrong spot,is there any free play in the pedal with the engine off? If not adjust the rod until you do, that should do it, I hope
Ken

If the rod between the master cylinder is too long, the brakes will lock up just as roofcam described. But, normally when the rod between the master cylinder and booster is too long the brakes will remain locked up and switching the engine off will have no effect. I've had to adjust that rod on many cars to fix this very problem but I've never seen vacuum make a difference. If the rod between the pedal and the booster is too long and it is applying pressure to the booster, it could cause the pedal to move down a little and lock up the brakes. The reason I didn't mention rod adjustment in my first reply is it sounded to me like you meant the vacuum was pulling down on the pedal as if a ghost were sitting in the seat pressing the brakes...I might have been too sleepy last night wheI read it though. Can you actually see the pedal move down when you start it up? I would definately check those adjustments as the others said but if the booster pulls the pedal down by itself when vacuum is applied, and those rods are adjusted properly, you've got a bad booster.

Yes...the pedal pulls down as if a "ghost" was doing it. I've adjusted the rod 3 times and even set it so it was way too short, and then tried it with the master cylinder off the booster. It didn't make any difference.

Denny...I've been thinking that the check valve could be the problem. I'll check that some more.

Thanks for all the help/suggestions!! As always, I find a lot of help on this site.

the check valve is just there to keep the vacuum in the booster from sucking gas fumes into the booster when you shut off the engine. after reading your last post I think you have a bad booster (after all sometimes even puppy's die brand new )....

the check valve is just there to keep the vacuum in the booster from sucking gas fumes into the booster when you shut off the engine.

I agree. You can't really have too much vacuum

now denny I don't want to get into one of your vendettas with you but that check valve has nothing to do with operation of the booster the valve that is in question is the valve inside the booster that is moved by the application rod hooked to the brake pedal that is commonly refered to as spool valve some real early units used a poppet type valve but any rate the booster has to be disassembled to git to it and I don't think he is interested in doing that on a new unit I know I wouldn't be .....ted

I agree with Ted. Full engine vacuum will not cause the booster to pull down on the pedal. The booster sees full engine vacuum when the engine is running and the check valve's only purpose is to keep vacuum inside the booster when the engine is off. It's called a check valve...not a regulator. We have to remember that he did not have this problem with his old booster....this booster is brand new and diagnostic procedures that are meant to check out a worn out booster won't work on a booster that isn't built properly. More than likely something went wrong in the manufacturing process. P.S. I'm ASE certified in brakes, my folks and I own a shop, and I teach a high school Automotive Service Technology class. Now, I'm not the loudest rooster on the fence, but I can't wait to hear the outcome on this one....If I'm wrong on this I'll just chalk it up to a great learning experience and a lesson learned I reserve the Right to be wrong at any given time

Awesome...I'm going for my master certification in May. Too bad I'll never know it all wish I did though

I went to college over 4 years

Yeah and I though I was done with all that...that was till I got this high school auto tech job. I've got to take 5 classes over the next three years....not even automotive related

Discussed my problem with the seller, ProComp. It agrees that the booster is bad and likely was a manufacturing problem. I'm going to send it back and get an exchange unit.

Thanks again for the help. Can't believe how much I learn from just lurking on the site and seeing you guys at work (play?).

Can't believe how much I learn from just lurking on the site and seeing you guys at work (play?).

That's why I started hanging out here hopefully a new booster will fix you up.