Losing brake pressure...any ideas?

Check your brake booster hose to see if it's leaking anywhere. The hose runs from your manifold intake to under the car, attaches to a steel line that runs to the front of your car, up through the front firewall and is connected to the brake master cylinder.

You may have a leak somewhere.

Sounds exactly like the problem Cyboman had with his clutch (see "sinking clutch pedal" from June 7th).
If the pedal just falls to the floor but you aren't losing fluid then it either isn't moving the piston in master cylinder or the piston isn't pushing the fluid.
So it could be that the piston in the brake master is not returning sometimes - so the pedal has nothing to push.
It could be grit in the master so that fluid leaks past the piston cup.

Either way I think you need to get the brake master apart to find out.

Swendog had this same problem with his brakes - he replaced his master cylinder to fix it.

Thanks guys. I will see if there are any leaks.

Kats:

Agree with above. Providing booster is not leaking, I'd be willing to bet it's your Master bypassing fluid internally. I rebuilt the stock Girling calipers on my '74, installed stainless flex lines, and pressure bled the system. Still needed to pump the brakes. Not very confidence inspiring! I bought the 1" bore late model Ford master with the billet adapter from Pantera East ($250). It's like night and day. You may be able to buy the same type of master for around $100 and hack around adapting it. For me, it wasn't worth it. Marino is great, his stuff always fits. Period. You do have to re-work the lines at the master though because the modern master uses 3/16" standard double flares versus the "bubble" flares on the stock unit. Figure about $50 is miscellaneous fittings. I would recommend removal of the old non-adjustable proportioning valve and installation of a Wilwood adjustable prportioning valve going to the rear calipers. I left the stock shuttle valve and brake switch in place. Some guys remove this as well and put in a plain brass distribution block and a more common brake switch (one with standard threads) Since the stock calipers are comical, the proportioning valve will be set at zero bias (100% to rear). But while you have the system opened, it make sense to put this in if you ever plan to update to a real brake system with beefier calipers out back. If you DON'T install the master yourself, figure about 5-6 hours time for a COMPETENT mechanic.

Swen

Thanks Swen! That is great advice. Much appreciated.