Update:

I adjusted the parking brake. It was dragging and was hoping this was my problem. It was not. Went on another drive, and had the same problem. When I got home I lifted the car and none of the 4 wheels were able to turn. I checked the vent line and it was not kinked or blocked (at least from the outside). There was a small amount of brake fluid on a nearby fuel hose that looked like it came out of the banjo bolt on the MC.

I was able to bleed the brakes using the Motiv bleeder so I think that would eliminate a vent line problem, right?

I think the only thing left are the brake lines? I was sure the previous owner said he replaced them and they look new, but I can't think of anything else. Maybe he did replace them when he got the car (in '00 I think) and have gone bad as the car sat for a number of years.

Brake lines fail fron the inside->out. They look great on the outside but are swollen shut and deteriorated on the inside.

I realize that they fail from the inside out. I just wasn't sure if they would fail as the car sits. They looked newish with clean fittings and rubber. Is the swelling visible? I have a new line that I could compare it to.

Did cracking the bleeder release the pressure and allow the caliper pistons to retract. You mentioned you were going to try it, but did not see that you actually did.

The rubber lines will fail from sitting too long without proper care or exercise. Brake fluid will also absorb water by just sitting.

I'm hoping my steel braided lines will be OK. My car has been sitting for awhile, but I drained all the fluid out when I started my suspension project last year. I'm getting close to putting the new rear calipers on. I have not determined if I can use the stock rear hard line to the caliper or if I'll have to bend my own lines. Time will tell.

yeah the brake lines will fail while the car is sitting. Even steel braided lines will fail. What sounds really really strange is that more than one caliper is locking.

If you were able to bleed your brakes with the Motiv bleeder, and you did NOT have to crimp the vent tube, then the problem is a blocked vent tube.

Rusnak- That doesn't make sense to me. The vent tube is the line that goes from MC to the resevoir right? The motiv puts pressure into the system through the resevoir. If it was blocked or crimped the bleeder wouldn't work.

Nevermind Rusnak, I did a search on vent line. Mine is a '67 and does not have a vent line.

I tried to find a schematic for the '67 but Porsche parts only goes back to 1970. So I looked at the 356, and the individual brake cylinders have vent openings to outside air. The 1970 brake master cylinder res. has a vent tube.

Somehow you are getting a pressure buildup on the brake cylinders that is causing them to lock. I'd trace the cylinders, the brake lines, anything for a vent of some kind.

http://forums.pelicanparts.com/uploa...1212609856.jpg

http://forums.pelicanparts.com/uploa...1212609873.jpg

Update: After some down time I replaced the front and rear brake hoses. A few short test drives later I have encountered the same old problem. Not sure where to proceed from here. I am against haphazardly replacing parts with new ones so . . . any advice? I will trying to crack the bleeder valve next time I induce this problem. I haven't been able to do it thus far because I have had to wait and let the brakes cool and release before getting back to my garage to check things out. Rusnak - there is a brake line schematic if you follow the new Pelican parts diagram links. I have looked the '67 schematic over and don not see a vent line, maybe I am missing it.

Could the rotor possibly expand enough due to thermal expansion to wedge in the pads? If the pads are new and their retraction mechanism is not well lubricated, could their be clearance issues?

What kind of rotors are you running? Vented, solid, Carrera? The soild may grow more due to less cooling. The vented are wider and the Carrera are 24mm. If the rotor is too wide, maybe clearance issues?

I am stumped. I wish I could experience this car in person to help.

refresh my memory a bit here: you say all 4 wheels are locking right? Or do I have that wrong? If it's all 4 then you need to find out why there is a pressure build up that is common to all 4 brakes, which is what led me down the brake vent tube road.

If brake fluid was boiling locally in the caliper piston bore, would it build up the pressure in the system?

Yes, binding on all 4 wheels. The rotors are solid.

I think the brake fluid could boil if the calipers were dragging, like say the parking brake left partially "on" or adjusted incorrectly. Can you check the rear wheels and make sure that they aren't dragging?

Otherwise it seems that the problem in somewhere in the mc.

I adjusted the parking brake. No interference there.

there is a Porsche repair kit for the mc consisting of a new piston, boots, o-rings, etc. I think if you took your old one apart, you might see if maybe something looks wrong.
When the calipers lock up, does the pedal feel any different?

Think about it, what is the common link to all 4 brakes?
There are some hard lines, and maybe one of them is crushed? Maybe the mc piston is not fully retracting, letting the brake fluid pressure return to normal when the pedal is released.

Thanks for everones help so far. Great community! When the brakes are binding the pedal gets stiff. When it gets to this point there is no travel in the pedal. I will search the system for a bent or obstructed line. Short of any new finds there it looks like replacing the MC is the next step.

On older systems the reservoir cap is vented since the reservoir must be vented in some fashion. Does your brake pedal push rod that actuates the master cylinder have the correct 'venting clearance'? I forget the exact spec, but the rod needs a tiny gap (about 1mm) before it contacts the back of the piston. Otherwise, the MC could be actuated all the time.