I just completed a drum to disc brake swap on my 71 Nova. The booster and master cylinder are new. Everything looks good and bolted right up. The problem that I have now, and have been trying to solve for the past few days...is that I have no brake pedal. I pressure bled the brake system at all four sides. When the motor is off, the pedal builds pressure and gets firm. When the pressure bleeder is hooked up, the pedal builds pressure and is also firm. (like how it should be) However, when the motor is on...the pedal goes straight to the floor...and never gets firm.

It seems to have the symptoms of air being in the lines somewhere. I bled them 3 times to make sure that no air was present. Where else should I look or check for air? How can I get my brakes back? Any helpful information is appreciated.

Thanks

So-Cal Jack

On those firm brakes with the motor off, do they stay firm after repeatedly pumping the pedal? If so you are feeling friction and not hydraulic resistance, because after a few cycles the vacuum in the booster will bleed down to nothing, with a different pedal feel as you loose vacuum assist.

There is a one way check valve between the manifold and the booster that holds the vacuum inside the booster with the engine off. If you were to splice into the hose after that point you could measure the vacuum the booster sees (a long duration cam will kill available vacuum at speed). This will tell you if you need a vacuum accumulator to help a marginal vacuum, or an electric vacuum pump (found on all GM diesel pick-up trucks) for a really radical cam with no engine manifold vacuum.

If you had a vacuum leak like you are hinting at the motor would have an erratic idle. You can check the vacuum hoses with an unlit propane torch, but bear in mind this can be risky if your ignition leaks it could ignite the propane. If the RPM picks up with the added fuel you have a vacuum leak.

The first thing that comes to my mind is the hole you chose for your brake pedal push rod, and it's overall length. The power assist hole is further down as you do not need as much pedal effort. That results in a different length rod for manual and power brakes. I do not think you are pushing the plunger fully into the master cylinder (once again you can measure the line pressure, but few outside of a brake shop will have the required gauge, and it is a pain to splice into the line for a single test).

Big Dave

stupid question. did you bench bleed the master cyl?

Hey Big Dave, you from Team Chevelle? Where is the electric vacuum pump on the diesel trucks?

Hey Big Dave, you from Team Chevelle? Where is the electric vacuum pump on the diesel trucks?

The only factory installed belt driven vacuum pumps I ever saw were on some mid 70's Cadillac's. The early GM diesel pick up trucks with the modified Olds engine had a vacuum pump that was cam driven and was in the hole normally used by a distributor. Later models all used hydra boost systems.

Some diesel powered Caddy's and late model pick-ups with a diesel have electric powered vacuum pumps. Not all. I got one off a Caddy, but I can not remember the year. They are installed on every airplane made since WWII so you can find one cheap at an avionics salvage yard; only problem with that idea is they run on 28 volts.


Big Dave