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I'm tying to figure out why the brake pedal gets mushy in the '52 300. It only happens when the engine is good and hot and I've been driving for 20 or 30 minutes. When I try to stop it usually goes to the floor and locks on the toeboard, if not it requires a careful touch. Also when on an incline the pedal slowly creeps down.

All the wheel cylinders and master have been rebuilt. The master was also rebuilt 6 years ago along with the treadle vac. I've read about the compensator valve and diagpragm or cylinder wall air leaks. The mechanic says it's all working as expected and the fluid is full and clean so I had few clues. I figured rebuild the treadle vac again and reline the shoes. Then it occurred to me that the heat of the brakes and the heat of the engine are overheating the fluid. I'm thinking about switching the brake fluid to ATE Super Blue racing fluid and making a heat shield for the master. Any thoughts on how effective that may be?

-John

IMO, Unless you are using a brake fluid with an exceptionally low boiling point I think you have problems in the BTV and it needs to be gone thru. Unlike the V8s where a manifold is inches away, the 52 brakes are a long way from any heat source that could get the fluid that hot,

Ok. I will start there.

You would have to be overheating the fluid to about 300 degrees before you would have sponginess from that source. I would be looking more along the lines of the brake shoes being in poor adjustment--ie in need of the major, not just minor adjustment in order to get full shoe contact. This is described in the brake section of the service manual, and has been covered on the forum here many times.

BUT, if your pedal is sticking while all the way down, then it is time to build your Treadlevac again. When all is adjusted properly, the pedal will hardly move a full inch in normal braking.

ok. All new parts aand service of the brakes. But need a bit more history.
Didd the brakes work just fine for several years and several thousand miles BEFORE this spongy/sticking pedal problem occured???

Try the adjustments to the brakes. If that doesn't work then send the master cylinder and power unit to Ed Strain in Fla for rebuild. Keep us posted.

The brakes worked ok when I started driving the car a year ago. However before that it was not driven regularly. The few times it was driven were short and slow runs. They had gone out before so I believe the hydraulics have been rebuilt twice recently the last was in December 2014.

The brakes work but they don't hold. When you sit on a hill you have to hold the ebrake because the pedal will sink. When going down a hill you have to severe brake to shake off your speed, lift the pedal, then carefully brake to a stop. It's quite a task to judge the distance and pedal throw without locking the pedal to the toeboard. When it's hilly I have to take off the overdrive so I can engine brake.

I want to get the new Kanter BTV kit with the new diaphragm. A retiring restorer said the diaphragm or cylinder could be leaking. We spent a lot to get the hydraulics rebuilt in December too but the shop doesn't know what's wrong with it now.

Anyone is welcome to correct me if i am wrong but it seems to me that over the years and the literaly multitude of discussions about the BTV, thaat it is generaly accepted that the BTV should be rebuilt every 3 to 5 years and the entire brake system flushed and refilled with new fluid about every year.

Such a rigorous maintenance schedule is why many of us have converted to a modern power/master cylinder unit.

...generaly accepted that the BTV should be rebuilt every 3 to 5 years and the entire brake system flushed and refilled with new fluid about every year.

I think it's advisable in any brake system in a collector car that sees infrequent use to change brake fluid every 3 - 5 years. But to rebuild a BTV every 3 - 5 years; more like every 30 to 50 years if it's had regular use and clean fluid.