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Spongey Brake Pedal

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Twstnshout

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Unless I pump my brakes constantly, the brake petal goes to the floor. I replaced the whole brake system: master cylinder, all wheel cylinders, pads, turned the drums, new fluid. I bled the brakes when I did that.

I have finally got the car roadworthy except for the brakes. I will bleed them again, but are there any thoughts on additional issues to look for. It’s a 1955 BN2 survivor that hasn’t been registered since 1969.
 
I had a similar problem with my BN2. If you can muster enough pedal resistance, place something like a board between the seat and the pedal, keeping as much pressure on the pedal as you can. Let sit for a day or two. This worked for me. I don't know why, but somehow it must force air out of the system. If you can't build any pressure you have bad seal somewhere.
 
Wheel cylinder boots can contain some fluid that can get by seals, it doesn't take much leakage to cause a spongy pedal. The brakes on 100s are, well, weird (anachronistic?). Roger Moment wrote an article for Austin-Healey Magazine on their functioning but, even after reading it I couldn't explain how they work. They don't have the 'foot valve' used in the 6-cyl brakes that closes off the reservoir return line, instead using a hole in the cylinder that the piston closes off. I suspect that is a possible source of their spongy pedal (you won't lose fluid as it flows back to the reservoir). Note 6-cyl systems can be tricky to bleed as well; I usually pressure bleed then follow-up with the 'pedal exercise.'

For my car, I tried the 'no-name' M/C first, then sprung for the TRW ($$$). I'm not sure it helped much; I tried a lot of things before the 'board-on-pedal' did the trick.
 
I think Bob's on the right track - there is still some air in the system. Air compresses and fluid doesn't so if you have air in the system, it compresses when you step on the pedal - things get better while pumping simply because not all the fluid returns to the reservoir if you pump quickly enough and so some of the air is already compressed when you come down on our next pump, eventually becoming a firmer pedal (because the air is more compressed). In just a few seconds, after you quit pumping, the fluid does all flow back to the reservoir and then the first pedal stroke is again to the floor.
As Bob said, getting the all-drum system bled can be a battle - search the forum for several discussions about bleeding the 100-4s (I haven't had a 100 for a long time so can't help much with details).
 
About the bubbles that Michael's article describes, I have also seen it suggested to tap the cylinder(s) with a mallet in order to make the bubbles detach from the walls of the cylinders.
 
Of all the tricks mentioned above, what finally solved it was investing in a one-person bleeder. You can test for leaks and bleed with system under constant pressure, but I have to say that blocking the pedal under pressure for a few days seemed to help also for some reason.
 
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