Brake Pedal Seeps Down

[Ron H]

I had a disc brake conversion done on my '63 Plymouth about two years ago adding power brakes. When driving the car the pedal seeps down intermittently and won't return fully unless pulling the pedal back. Seems to hold for a while then same thing happens. A little pumping action will bring the pedal back sometimes. When checking the booster after car is off, removing the vac line, there's pressure. I have no indication of vacuum leaks far as I can detect. Any advice? Thanks

 

[HALIFAXHOPS]

Can be the master cylinder bleeding back internally or the adjustment on the push rod is off.

 

[Ron H]

Can be the master cylinder bleeding back internally or the adjustment on the push rod is off.

Any quick thoughts on how I could identify this? Thanks.

 

[khryslerkid]

Without the engine running (no vacuum) pump the brake pedal a couple of times then hold pressure on it. As you hold pressure feel if it starts to go down. Let off ever so slightly but keep pressure on the pedal and see if it drifts down. If it does take note to how far. If it's a lot then the master cylinder is leaking past internally.

You could do an inspection behind the master cylinder by taking it loose from the booster and see if there is any brake fluid present. That's a sign of the master cylinder leaking externally.

Check behind all four wheels and hoses for any signs of brake fluid leaks.

 

[dadsbee]

New or old return springs on the drum brakes?

 

[HALIFAXHOPS]

KK nailed it. The adjustment rod is different on the cars. Biggie is that it returns out of the master if that makes sense.

 

[Ron H]

I gather there's a sweet spot on the rod adjust between the MC and the booster; if I follow you guys, this could mean the booster is pressing on the MC when the pedal is not being pushed huh? The brake kit instructions didn't seem to talk about this, but tell ya, the shop that did the install left a lot to be desired. The pedal was left so high I coulda used it better as a hand brake. I ended up having to rework the rod, shorten it, and a bud machined a new eye bolt as the kit piece was actually bent, aside from a few other shortfalls. One of them was the huge gap left where the boot sets into the firewall. I installed a thin gauge plate btw the booster and firewall with a 'seat' for the boot to set in. All of the rework was on the firewall side not messing with anything btw the MC and booster. Thinking on your tips, wonder if this may have messed with the position of the booster? Anyway wonder if the prep work was done or done right such as bench bleed of the MC. I had re-bled the brakes twice and checked the prop valve in the hunt to fix the problem. As for the comment on the rear drums, the re-do on those have maybe 3 or 4k on them. They seem adjusted as should be. There's no loss of fluid anywhere MC is full.

 

[HALIFAXHOPS]

Bet it is slightly pushing the brake plunger in. Can cause the issue.

 

[Dipstick]

A push rod that's too long will cause the opposite, a hard pedal.