I have a 93 5.2L 2WD Dakota. Had major brake problems when I got it. Rear brakes were replaced but the PO stopped there. The front calipers and rotors were trashed and it looked like someone hit the master cylinder cracking the reservoir and bending the booster. I have replaced the master cylinder, booster (twice), calipers and rotors, bled the brakes and now have plenty of pedal pressure. However, when I start the engine, the pedal goes to the floor. Turn off the engine and pressure returns. I re-bled the brakes and same result. Good pedal pressure until I start the engine and pedal goes to the floor.

Any idea what is going on? Haven't checked the vacuum pressure yet but not sure if that could even be the problem.

Thanks for any ideas at all.

Todd

 

try this: Functional Test:

1. With engine off, depress brake pedal several times; then, depress the pedal hard and hold that pressure for 15 seconds. If the pedal sinks, the master cylinder, brake line, or brake caliper is bad.

2. Start engine with pedal depressed. If the pedal sinks slightly, the vacuum booster is working. If the pedal height does not vary, the booster or check valve is faulty.

Leak Test:

1. Depress the brake pedal with the engine running, then stop the engine. If the pedal height does not vary, while depressed for 30 seconds, the vacuum booster is fine. If the pedal rises, the booster is faulty.

2. With the engine off, depress the brake pedal several times using normal pressure. When the pedal is first depressed, it should be low. On consecutive applications, pedal height should gradually rise. If the pedal position does not vary, check the booster check valve.

Check Valve Test:

1. Disconnect brake booster vacuum hose at booster.

2. Start engine and let it idle. Vacuum should be available--if not, the check valve is not working correctly. Replace the check valve and retest.


if all that tests good you got air still (I like to gravity bleed for a while before I really bleed them but that's me)

 

OK, here's what I found:

1) Brake pedal holds pressure for as long as I want to hold it down. Even hard.

2) With pressure on the pedal, I start the engine and it goes almost to the floor. About 1/2" from floor.

Leak Test:

1) With engine running, pedal goes about 1/2" from floor. Turn off engine and it stays there for 30-45 seconds.

2) Pedal has pressure with the engine off and does pump up some on consecutive pumps.

Check Valve Test:

1) With vacuum hose disconnected, the brakes have good pressure and pedal height with engine running or not.

2) Not sure what I'm looking for in step 2.

I did notice when I put both new boosters in that the brake lights stay on as if the pedal won't come all the way back up. There is no way to adjust the push rod length. Went back to the store where I got the new ones and checked the push rod length on the old one and it is exactly the same as the new one. It almost seems that the pedal is not returning all the way up.

Still a little baffled.
Todd

 

On the brake lights - I do not KNOW this is the problem, but be sure to check the orientation of the rod - on my Cougar, if I turn it 180*, it'll not turn on the brake lights until almost all the way down (!!). On my old 1967 Chevy, it'd turn ON the brake lights even with no pressure on the pedal when the rod was 180* off.

RwP

 

Man, I thought you had it but rotated the push rod 180* and no difference. Just for the heck of it I measured pedal heights both ways and they were the same no matter which way it was rotated. Anyway, great idea, I wish it had worked. Sometimes it is the easiest, most obvious things that fix problems like this.

Oh well, thanks,
Todd

 

make sure the switch near the pedal didn't get broke. Sounds like still air in the system, did you bench bleed the master?

 

Yes I did bench bleed the master. I'm going to bleed the lines again. Do you think that might make the pedal return any higher?

 

Pulling down like that, as I understand it, is 100% the booster.

However, I'm not an engineer - don't even play one on TV! - and my expertise is electronic / computer (being a Certified Electronics Tech for about three decades now ...)

For bleeding, to make sure, you may want to use a vacuum bleeder. I use this one from Harbor Freight (runs off your air compressor).

RwP

 

Well I called my friend back over and bled the heck out of them. Went through 2 bottles of brake fluid and all kinds of cloudy stuff kept coming out of the rear lines. Once it started flowing clean the brakes worked fine. What I didn't realize is the pedal does feel a little soft but still stops the truck just fine. Must just be the nature of the beast.

Thanks to everyone for all the help.
Todd