My brother took the truck this morning and the brake pedal locked up on him. I test drove it after and the pedal seemed fine and then it locked up on me. There are no leaks in brake lines and the brake fluid remains at the correct level. I turned the truck off and pumped the pedal. Usually the pedal gets hard after a couple of compresses while engine is off. But the pedal just kept on pumping effortlessly for at least 20 times.

My conclusion was that the vacuum booster is bad. Any thoughts??

 

Check for leaks again. Look at all of the brake fittings, and look at the calipers for evidence of fluid stains underneath them.

It isn't the vacuum booster if the pedal pumps effortlessly with the engine off. You've got a fluid problem, caliper problem, or a master cylinder problem. With the engine off the pedal should be hard always. If it took you a few pumps to get a hard pedal before, whatever's broken was already on it's way out.

 

I looked under the truck and I didn't see any leaks in the system. And if there were, the master cylinder level would have gone down, right?

I'm sorry, I shouldn't have said effortlessly. It pumps kind of(not too) hard but not easy and not all the way to the floor. The pedal stops about half way down, but it just keeps on pumping. With the engine off, normally it took about 1-2 pumps until the pedal was rock hard. As far as I know thats how a proper vacuum booster should work.

Thanks for the help!
snapon

 

Check the rear wheels too. One of the wheel cylinders might be leaking, and it make take a while to leak down where you can see it.

What do you mean the pedal locked up? It got rock hard and wouldn't go down, or it went to mush and dropped to the floor.

Could also be the hoses breaking down, the rubber inside can break off and clog the line up.

 

I replaced the wheel cylinders myself about a month ago, I checked them and they aren't leaking.

The pedal itself is locked up hard while in the uncompressed position while the truck is running.

 

check the front calipers to see if they're releasing.

 

The pads aren't touching the rotors, so I assume that they are releasing.

 

The vacuum booster has ZERO effect with the engine off. So if it's different now then it was before and the engine was off in both situations, you've got a fluid problem, not a booster problem.

I'd be willing to bet it's the master cylinder.

 

The booster will hold enough vacum so that you should be able to press the pedal a time or two and then the pedal should feel like your stepping on a brick, hence the purpose of the booster. If "the pedal locking up" means that no matter how hard you press the pedal, while running down the road, it still feels like your stepping on a brick then it's more than likley the booster.

 

Yes and no - could be the booster cut it could be the vacuum pump as well. Actually I'm leaning towards the vac pump, as that's just what mine did when it was on its way out - not enough vacuum if I stepped on the brake pedal even twice in a row, but drive a few miles without using brakes and she'd eventually build up enough vacuum to assist in another brake apply...

 

I am sure this isn't it but it is worth checking. I tend to agree with LCAM. But, have you checked the pedal under the dash? Make sure there aren't any obstructions on the actual pedal itself. Make sure none of the rods are bent or make sure the the whole assy is operating correctly? I know I have lost the clutch pedal when the pin and grommet break. The clutch pedal at the top comes off the whole bracket. Aggravating when this happens at a stop light....LOL Anyway, like I said, I am sure that has nothing to do with it. Just me thinking outside of the box so to speak.

John

 

There have been a few in the past that the rubber brake line collapses when braking and doesn't let fluid flow back.

I think your problem is either that or a problem with the master cylinder.

 

Hey guys, I'm still having trouble figuring out what's wrong with my brake system. I have replaced the master cylinder, vacuum booster, original calipers, rusty brake lines, and yet the same problem-nothing changed, nothing different from before. I have just about had it with this thing. The line to the booster has vacuum, there are no leaks in the entire system, the master cylinder is full, the master cylinder and booster are new, and the calipers are not stuck.

I do feel kinda stupid for basically replacing the entire system for no reason but any one have any thoughts here???

 

How much vacuum at the booster line? Also how quick it builds up? Weak vacuum pumps will often pull the proper amount of vacuum, but it takes them a while to get there, which means they can't keep up if you use the brakes often...

 

When my pump started going out I noticed that it would take forever for the valves to operate on my air conditioning. It would direct all of the air to the floor for 5-10 seconds and then finally the air would start blowing out of the dash vents.