Soft brakes when engine is running.

[bohochg]

Have been helping a friend get his brakes sorted out on his 71 Charger SE, with 318 engine(stock) and power disc brakes.

He replaced everything in the brake system from the power brake booster all the way back to the rear brake cylinders about 5 years ago.

Trying to get the car back on the road.

We can get a firm pedal after bleeding the brakes, but as soon as he starts the engine the pedal goes soft for a couple inches or so before getting firm again.

Attached a power brake bleeder to the master cylinder, with no pressure drop in the system. Not seeing any leaks

Flushed system with new DOT 3 fluid

Verified the clearance on the rod from the power brake booster to the master cylinder.

Rear brake shoes adjusted for slight drag on the drums

Bench bleed the master cylinder couple times.

Have tried a different master cylinders and power brake boosters.

The brake warning light is on all the time, but that could be an electrical issue.

Any suggestions would be appreciated.

 

[Don Frelier]

Try driving it, I've seen the pedal improve after everything seats in a bit.

 

[RJ Squirrel]

A tiny leak somewhere that is letting air in.

 

[eldubb440]

sounds like normal power brakes to me

 

[OMTWF-72SE]

Vacuum attached to manifold? When you checked vacuum did you check thru the check valve? They need 16 atmospheres for a good pedal and my 72 is only good for about 3 full depressions or 1 really long stop light before its basically a slightly better than manual pedal. I've had 3 72s, with diff engines. Vacuum vacuum vacuum. And just wait till you hook the vacuum heater valve octopus up, you can have brakes or you can have good vent control but not both

 

[Mopar Hunter]

The brake warning light is on all the time, but that could be an electrical issue.

My 69 Charger acted like that, found out it was the sticking proportioning valve.

 

[OMTWF-72SE]

I read past the stuck light but agree with mopar Hunter. There is a spring plunger switch on the proportioning valve that creates a ground when pressure is low or if it fails or if the valve was allowed to dry out while off the car. You'll have to take it off to fix. Could also be a short to ground from the parking brake switch. I called Dr diff and just replaced my entire assembly. Got tired of jacking around with it.

 

[OMTWF-72SE]

I saved this breakdown on what is replaced if you rebuild

https://board.moparts.org/ubbthreads/upload2/7489638-poportioningvalve.jpg

 

[Geoff 2]

It is normal for the brake pedal to 'sink' slightly when starting a car with boosted brakes.

 

[bohochg]

We are still having issues with soft pedal.

I'm thinking air must still be getting into the system somehow.

Checked & remade all the fitting connections.

Pressurized the system to 40psi with a power bleeder attached to the master cylinder.

Watched as it held pressure for over an hour, but again at only 40psi.

No visible leaks.

No loss of brake fluid.

When bleeding the brakes we are getting good flow from all 4 corners, so I don't think it's the proportioning valve.

Could there be an issue with the front calipers?

Going to try these brass flares next.

Anyone have any experience with them.

 

[Geoff 2]

In the original post, you say the pedal moves once the engine is started, but then gets firm. Might be an internal problem with the booster.