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Brake pedal won't return

skunked

skunked

phat block everything
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as the title states, I having an issue with my brake pedal returning after it's been depressed. I cannot force it back up, brakes are definitely dragging until the engine is shut off. This is a fresh build with a late 80's master cylinder and booster (new) with a wilwood proportioning valve. 3/16" brake lines with stock Dana 60 front brakes and stock 14 bolt drums out back. The axles were used previously on my old build so I don't think the issue could be with either axle.

Some brief research on the interwebs has pointed to a bad booster? I should also mention my booster is getting vac from the FiTech EFI "booster" port.

image.jpeg
 
So,the brakes drag until the engine is shut off ?...sounds like the booster is "self applying" the brakes...have you noticed once the pedal returns with the engine off,if the brake pedal applies by itself when you start it back up again ?...

I haven't had a booster do that before--only issues I had with them is one Buick I had,there was a "delay" if you stomped on the pedal fast,like it refused to go down right away (scary in a panic stop situation!)--never did find out why or fix it,I just drove very carefully..others I had go bad simply lost power assist when the diaphram failed,and one I installed on my '75 K5 2wd truck was off a car,and it was SUPER sensitive,you barely touched the pedal and it would put you thru the windshield...that one too,I learned to live with..

I had a master cylinder leak into the booster once too,and enough fluid built up in the booster to render it in-effective,like it had a hard pedal but little braking action,until I siphoned all the brake fluid out of it..
 
Do you have a one way valve on the booster?? Sounds like you are pulling vacuum in the booster.
 
diesel4me said:
So,the brakes drag until the engine is shut off ?...sounds like the booster is "self applying" the brakes...have you noticed once the pedal returns with the engine off,if the brake pedal applies by itself when you start it back up again ?...

I haven't had a booster do that before--only issues I had with them is one Buick I had,there was a "delay" if you stomped on the pedal fast,like it refused to go down right away (scary in a panic stop situation!)--never did find out why or fix it,I just drove very carefully..others I had go bad simply lost power assist when the diaphram failed,and one I installed on my '75 K5 2wd truck was off a car,and it was SUPER sensitive,you barely touched the pedal and it would put you thru the windshield...that one too,I learned to live with..

I had a master cylinder leak into the booster once too,and enough fluid built up in the booster to render it in-effective,like it had a hard pedal but little braking action,until I siphoned all the brake fluid out of it..
Click to expand...

Correct, once I apply the brakes they stay at the floor with the brakes applied until the truck is turned off. At that point I can force the pedal up or it will return in 5-10 minutes.

frankin5 said:
Do you have a one way valve on the booster?? Sounds like you are pulling vacuum in the booster.
Click to expand...

I just ran a hose from the FiTech to the booster port.
 
I've used this FiTech port on other cars/trucks without issue :dunno:

image.png
 
Needs the one way valve or yours is defective. It is the white piece of plastic that plugs into the booster at a 90 degree angle.
 
Guess the easy check is plug the FuTech port and apply the brakes without vacuum assistance
 
Manifold vacuum IS what you get at the base of a carb, so I assume that the Fitech is the same. And the vacuum booster is designed to use manifold, or engine vacuum. I would suspect that there is a problem inside of the booster. You said that the booster is new, I would suspect it first, since it helps actuate the brakes.
The one way valve at the inlet is to hold some vacuum after the engine dies, for emergency braking.
I would guess that there is a problem with one , or both of the diaphragms in it. (Assuming that it is a dual diaphragm)
Did the master cylinder return decently when you bled it? On the bench or the truck. I would guess so, which would mean that the return spring is in it.
 
I guess the one way valve makes sure that it holds vacuum even when there is no much in the intake, such as WOT, otherwise you would have to wait for the vacuum to build after letting off of the gas to get boost to the brakes.
 
I'd guess the booster is flubbed up internally,if hooking the vacuum source to another manifold vacuum spot does not change anything..try that first..
Also ensure the brake pedal has a bit of free play before it applies the master cylinder--if none exists,after the brake fluid heats up, they can stick "on"...you need room for expansion..

That check valve at the booster's vacuum supply hose only prevents the vacuum from being lost back into the intake after the engine shuts down,so you'll have 2-3 "boosted" applications of the brakes with the engine off--it would still stop normally while driving even if it had failed,you may notice a slight decrease in power assist too,but it wouldn't make the pedal get stuck "down"..

If you see the brake pedal apply itself after starting the engine,something internally in the booster has probably failed,there are other valves inside it that balance the vacuum applied to the master cylinder side of the diaphram vs the atmospheric pressure on the pedal side in there and/or the diaphram may have a ruptured...

If you had to bleed the brakes and the pedal got pushed all the way down,it may have damaged the booster internally..

A quick test to see if the booster is working as far as power assist is to depress the pedal several times to use up any residual vacuum in the booster--then hold your foot on the brake pedal and start the engine--the pedal should go down if its "boosting"...but seeing your pedal is not returning I suspect its an internal issue with the booster..
 
You should be able to work the brakes with the line unhooked. They will just be really stiff.

Is you return spring under the dash attached ?
 
kgblazerfive said:
You should be able to work the brakes with the line unhooked. They will just be really stiff.

Is you return spring under the dash attached ?
Click to expand...

Disconnected the vacuum, pedal returns as it should.

Spring is connected however like I mentioned before, I can't force the pedal up let alone a little return spring (engine on and vacuum connected).
 
Its the booster. There are two chambers in the booster if its a single booster. There is a flexible diaphragm between the two. There are two valves attached to the brake rod. When the pedal is at rest, one valve is between the two chambers and its open. That allows the vacuum to equalize between the two sides. The other valve, is between the back side and the outside air, and its closed.
When you press the pedal, the valve between the two closes blocking the back side from getting anymore vacuum.
At the same time, the other valve opens allowing outside air into the back side.

The vacuum on the front side pulls the diaphragm forward due to the atmospheric pressure filling the back side.
When you release the pedal, the outside air valve is suppose to close and the valve between the two sides is supposed to open allowing the vacuum to equalize between the two halves.
When you release the pedal, for some reason the internal rod with the two valves on it is not returning, which acts like you are holding the pedal down.

Something is binding somewhere. Since its a new unit, I would tend to suspect a manufacturing defect. However, before you take it back for warranty, I would make sure the mount where you bolted it to the firewall is a good fit and not warping something when you tighten it down.
Also make sure that the pushrod from the brake pedal to the booster is pushing fairly straight in and not at an extreme angle.

I saw something like this once, when a booster got a hole in the back chamber. But that caused a big vacuum leak, and the brakes would apply themselves as soon as you touched the pedal.
 
Fordum said:
Its the booster. There are two chambers in the booster if its a single booster. There is a flexible diaphragm between the two. There are two valves attached to the brake rod. When the pedal is at rest, one valve is between the two chambers and its open. That allows the vacuum to equalize between the two sides. The other valve, is between the back side and the outside air, and its closed.
When you press the pedal, the valve between the two closes blocking the back side from getting anymore vacuum.
At the same time, the other valve opens allowing outside air into the back side.

The vacuum on the front side pulls the diaphragm forward due to the atmospheric pressure filling the back side.
When you release the pedal, the outside air valve is suppose to close and the valve between the two sides is supposed to open allowing the vacuum to equalize between the two halves.
When you release the pedal, for some reason the internal rod with the two valves on it is not returning, which acts like you are holding the pedal down.

Something is binding somewhere. Since its a new unit, I would tend to suspect a manufacturing defect. However, before you take it back for warranty, I would make sure the mount where you bolted it to the firewall is a good fit and not warping something when you tighten it down.
Also make sure that the pushrod from the brake pedal to the booster is pushing fairly straight in and not at an extreme angle.

I saw something like this once, when a booster got a hole in the back chamber. But that caused a big vacuum leak, and the brakes would apply themselves as soon as you touched the pedal.
Click to expand...

Thank you @Fordum for the detailed explanation.

I bought this master cylinder and booster combo through rock auto months ago so I'm probably screwed on a warranty claim.
 
Yep, probably so. Check for a leak on the back side of the booster. A small pinhole or crack might be fixable. Also try applying the brakes, turning off the engine, and listening for air rushing in somewhere.
If its just a sticking internal valve, whanging on it with a rubber mallet while its locked up with the pedal released might free it up. If its just stuck from sitting, it might free up and be fine with use. If its a broken internal return spring, then that would not help.
But just a broken spring would not keep applying power when you release the pedal. Something is allowing atmosphere in the back side, or not allowing vacuum in when you release the pedal.

Also, if all else fails, you can probably get just a booster cheaper than the combo you bought last.
 
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