Weird power brake problem...

[jenkins71]

Hi guys.

71 Charger, 440, power brakes, factory discs up front, factory drums in rear.

My power brakes are heavily boosted, which is weird enough, but sometimes, upon applying the brakes and moving the pedal down pretty far with my foot, the boost goes away and I am left with a rock-hard, seemingly underpowered brake pedal! Pretty much only happens when slowing down for a stop, and doesn't always happen. Also, if I release and sort of "pump," the brakes will work normally.

Last night while driving on empty streets, I had the "dead rock hard" pedal sensation and decide to not pump it and see how well it stops when the pedal gets like that. Well, it doesn't stop well at all. Rolled into the middle of the interesection.

I have bled the brakes twice trying to fix this, and also replaced the master cylinder. The problem still exists!

Any ideas what the hell is going on?

Thanks!

 

[747mopar]

Sounds like a vacuum problem, check the hose, check valve etc. Also try a vacuum pump with a gauge on the booster to see if the diaphragm is bad. You could also route a hose threw the firewall for a temporary vacuum gauge and see where the vacuum is when it's working normal and when it's failing.

 

[jenkins71]

How do you check the booster with a vaccum pump and gauge? I have a mity-vac...

 

[69Runner]

How do you check the booster with a vaccum pump and gauge? I have a mity-vac...

Pull a vacuum, valve off and check the gauge for needle movement.

 

[Shorthorse]

Did you bleed the new master cylinder?

 

[mopar 3 B]

How do you check the booster with a vaccum pump and gauge? I have a mity-vac...

Pull the vacuum hose from the manifold and adapt it to your mity-vac need to install a tee for a vacuum gauge. turn on the mity-vac if the gauge shows a vacuum pull the booster is likely good. Pinch or close the line off to the mity-vac and the vacuum gauge should not move. If the gauge looses vacuum the booster diaphragm or can has a hole in it and will need to be replaced.

 

[jenkins71]

Did you bleed the new master cylinder?

Yes, I bench-bled the master (or, rather firewall-bled)

 

[Bb70charger500]

I would say booster . Let use know how the test go's