Bleeding Brake Woes

[jsz]

I am just finishing up my restoration of a 1978 Spider and for the last month have been trying to bleed the brakes. Here is what I have done. The calipers, rotors, rubber brake lines, proportioning valve, speed bleeders and master cylinder are new. The existing brake lines, reservoir and booster are reused. Prior to disassembling the car the brakes worked very well. So here is what I have tried: pressure bleeding, gravity bleeding, vacuum bleeding, removing the bolt to the proportioning rod and lifting the rod up, have the rear end higher, bleeding MC prior to install and etc. I have been so frustrated that I took it to three different garages without any change in the brakes. I do have brakes, they go to the floor, can not lock them up, I remember these as much more responsive. The front brakes have pin head bubbles at time when bleeding, the process has been repeated 10 times to no avail on bubble issue. Back is solid fluid. No leaks, have rechecked fittings. I am leaning towards the master is bad. Any help or advice is most appreciated.

[azruss]

If you still have bubbles after bleeding 10 times then you are sucking air in somewhere. are you getting air bubbles on both front wheels. If so, that might indicate an MC problem. If you only have it on one side, then you need to look at that circuit only. even if you have an air leak, that should just make the brakes spongy and not necessarily go to the floor. I would say its a good chance the MC is defective. Do the brakes go to the floor with the motor off. This could indicate a bad MC or the booster rod between the MC and booster is missing or horribly out of adjustment. I'm not sure you would have any brakes at all if that was the case.

[jsz]

Thanks for the response. Yes both sides on the front show minute bubbles. When the brakes are pressed without the car running, they are firm and move about 2". When started they move close to the floor and will not lock up. The booster was not removed during the restoration, so thinking the adjustment has not changed from when it previously worked.

[jsz]

With car off, with moderate pedal pressure, the pedal continues to gradually go down. I am convinced the new MC is bad. Now making plugs for each circuit and test, if the pedal continues to depress it is leaking by internally. We will see how this goes

[seabeelt]

Did you bench bleed the MC before you installed it. If not, it may be that there are still some air pockets in the MC.
R/

[htchevyii]

I had a heck of a time getting all of the air out of the master cyl after I replaced it. It might need to be bench bled it you didnt do it. It can be done on the car by disconnecting the lines and running hoses into the reservor. I haven't tried a pressure bleeder, but if you search for garden sprayer pressure bleeder, there are plenty of plans.The Russel speed bleeders are sweet.

[rlux4]

I bench bled mine when I replaced the MC. The key to getting all the air out is to turn it to a couple of different orientations so that any trapped air can rise. Darn hard to get all the air otherwise.
Ron

[kilrwail]

It sounds like there’s air in the master cylinder. Bench bleeding it while it’s in the car is messy and difficult and you’ll still have air when you’re finished. Here’s an easier way. Patience and persistence are required, but it works.

Stuff a lot of shop towels around and under the m/c to catch drips. Crack both output lines at the m/c about 1/8th of a turn. Depress the brake pedal ONCE and hold it down with a person or a stick of wood jammed into the front of the driver’s seat. Tighten the lines at the m/c. Release the brake pedal to the rest position. Keep adding fluid to the reservoir as required to maintain a full level. Repeat as required until the pedal is very firm with the engine off. Be careful to only depress the pedal ONCE each time, or you will suck air back into the system!

[jsz]

Thanks for all the great thoughts. I have bleed the the MC in a vice prior to installation. I have plugged each of the circuits at the MC and the pedals moves about 1.5" and locks up. Now in the process of isolating the circuits that are causing the bleeding issues. Talked with a client who restores Porsche's at he has had that problem on rebuilt calipers. He suggested I take them apart , (suspect circuit), clean and reassemble. I am getting further detail on this issue from him. All the logical things have been explored and re explored, with 3 masters, (2 new) put on to no avail. A bit frustrating none the less.