Lightn95

I put a 5.3 from a Denali in a 53 Cadillac. I used a hot rod power brake setup. I pulled the vaccum from the back port on the truck manifold. When the motor is off, the brake pedal is firm. As soon as the motor is running, the pedal goes right to the floor.
I've bled the brakes with the Motive brake bleeder, there's no air in the lines ( in theory ) all 4 wheel cylinders are new. There's no leaks anywhere.

why would I lose pedal pressure when the motor is running? Im thinking Master cylinder or booster is bad?
any help would be appreciated!
this Cadillac is fighting me every step!

Creekin

Are the brake shoes adjusted properly?
sometimes having a second person helping bleed brakes is the best way

gofastwclass

iTrader: (2)

You said you had four new wheel cylinders so I assume you're running drum brakes on all four corners. I wouldn't run all drum brakes on anything but your call.

As Creekin said, are you sure the brakes are adjusted properly? Drum brakes typically have a lot more adjustment range than disk brakes. If the mechanical adjusters aren't set properly or the brake springs and shoes aren't installed properly you will have little or no pedal. Depending on how far off the brakes are you may barely touch the linings to the drums and the vehicle won't have enough brake to hold itself still in gear with the pedal on the floor.

Lightn95

Yeah it's drums on all 4. For some reason I get better pedal pressure when the car is not running. As soon as the car is on, and there's vacuum there is no pedal pressure. It goes right to the floor. Even if you pump the brakes. There's no leaks and no air in the lines. I'm starting to lean towards to booster. Not sure if it's to small or big?? Or if it's the wrong diaphragm. It's a 8" GM style booster.

Creekin

Booster size shouldn't be a problem, does the pedal fade away with the car off and no vacuum assist

Also one other thing to check is if when you swapped out the wheel cylinders did you get the little push rod on either side seated in the shoes, good time to check adjustment too
I've had it where pedal feels good in the shop till you drive it, bring it back in and rebleed the system RR, LR, RF LF (usually) with a helper and readjust
I try to exhaust all the simple easy stuff first before I would swap any other parts

G Atsma

Early 50's brakes are crap. Put some discs all around if you want a safe car, with the added power it now has.

LLLosingit

iTrader: (4)

You answered your own question, When the car is running you have the brake booster doing all the work and when the car is not running and has no vacuum going to the booster your foot has to do all the work. There is a huge difference in pedal pressure needed between the two.
If you plan on driving the car much at all then ditch the front drums at least. Disc conversions aren't that expensive.

Retrorod

You have a hydraulic problem not a booster problem. I’d start looking at adjustment and air in the system first. Air compresses, fluid doesn’t not compress. If you think the adjustment is good I’d start with a good pressure bleed.

Sure disc brakes are much better but that 53 Cad will stop ok with drums if the system is working. An 8 inch booster is ok but a little small.

Lightn95

I talked to a few brake companies and I ordered a 9" dual diaphragm booster. A few companies have said an 8" is a little small. Once I get the new booster in, they also recommended checking the distance from the push rod in the booster to the plunger in the master cylinder. They need to be just about touching. No more than .020" away from each other.