Dragging Brakes

The mystery has been solved! The dragging brake problem was due to and improper rebuilding of the booster. When the engine compartment heated up, the servo piston didn't retract properly when the brakes were released because the piston assembly wasn't pre-loaded properly when reassembled. Thank you to all who responded.
Chuck

Well the saga continues but progress has been made. I now know that the problem is definitely vacuum based but does not originate at the reaction valve (replaced). I disconnected the main vacuum line from the engine to the check valve and presto, all four brakes released. So it is now my opinion that once the booster/system heats up, a vacuum leak develops (engine rpm's increase when the engine is hot/brakes are dragging so this is consistent). When I hit the brakes, this admits atmospheric pressure to the servo diaphragm. When I take my foot off, there is not enough initial vacuum in the system to retract the piston in the servo because of the leak. Does this sound plausible to anyone who knows the S3 braking system?
Thanks,
Chuck

I appreciate all who have contributed to this forum. After going through all suggested diagnostics, I have been unable to pinpint the problem. I do believe Dave is onto something with the reaction valve theory and I have ordered the piston lubricant that was suggested in the article. I will report back when I have tried that option. Again, thanks to all.
Chuck

More:

http://www.mossmotors.com/forum/forums/11205/ShowThread.aspx

I think there is a big difference between rapid lock and slow drag due to heat expansion of fluid.

Just another bit.

The quick way still is to see where the pressure is.
M/Cyl, or booster.

http://www.newhillgarage.com/Page10.html
"If you find that your brakes are not releasing, the first thing is to establish if the problem is at one wheel or at all the wheels. If the car is drivable, this may be obvious if the car pulls to one side under braking. In my case, since the car was in my shop, I simply got the wheels off the ground and tried to turn them. I found that both front wheels were locked.

If only one wheel is locked, it may be due to a sticky caliper or a rubber brake line that is so old that it collapses internally and becomes a sort of one way valve. In my case, I pulled one of the vacuum lines leading to the brake booster unit and the brakes immediately released. From this I was able to discern that the problem was probably with the vacuum booster system rather than with the hydraulic system.

It turns out that the problem resided with the mechanical interface between the master cylinder and the reaction valve. In the setup used on my E-Type, the reaction valve is mounted directly to the end of the master cylinder.
During my rebuilding of the master cylinder, I found that there is a small metal cylinder fitted at the end of the master cylinder. This cylinder moves outward from the master cylinder when hydraulic pressure is applied, pressing on the reaction valve, and causing it to actuate, which introduces vacuum to the brake booster. When you let up on the brakes, the cylinder is supposed to retract, which allows the reaction valve to close, and removes vacuum from the booster. In my case, the cylinder was not retracting properly, causing the vacuum to remain on the booster, thus locking up the brakes.

When I rebuilt my master cylinder, my rebuild kit contained a small o-ring for the actuating cylinder, which I duly replaced. The o-ring serves to seal the cylinder so hydraulic fluid cannot leak by it. It seems that the o-ring sealed a little to well and caused the cylinder to hang up in the extended position. After I eventually discovered this problem, I had to "exercise" the cylinder by hand quite a few times until it was broken in enough to retract on its own. "

Had a forum issue so I am late to the party--if this is part of a restoration I would check all vac lines to make sure they are routed correctly and that they are secure--furher I would make sure that vac line was used and not just rubber hose.

Check all the vacuum lines that go from the engine to the tank, reaction valve and both sides of the booster. One of the hoses on mine was swollen shut inside and no air was getting through.

Thanks Dave, I will try out your approach and report back.
Chuck

Oh.....and set the park brake or have it in gear when you do this.
Helped out a guy at the ABFM in Bellevue last month or so, with a TD, locked up, couldn't get up the hill, popped the fluid at the LF w/cyl, even tho I'd asked him to have the e-brake set, I ended up holding the wheel to keep it from rolling down the hill while he set it.

A test is to crack the line at the m/cyl when they lock up.
If it goes "poof" into a rag, it's the m/cyl.
If it doesn't, crack the line on the brake side of the booster.
If it goes "poof" into a rag there, and not the m/cyl, it's the booster.

Thanks for the tip William. I jacked up the car and unfortunately all 4 brakes are dragging. Also, there is plenty of free play when the car is relatively cold. It's just when the car gets hot, the pedal hardens as if the booster is inoperative which I think, could be the result of the malfunctioning reaction valve.

Chuck

It is possible that there is insufficient free play at the brake pedal, there is an adjustable eccentric pivot on the aluminium housing to allow it. I had an E a year or so ago that had been restored and cheap aftermarket calliper kits were used, the rubber was very soft preventing the pistons from returning properly. It was just one wheel so (at the time, not realizing the cause) I only rebuilt one rear calliper, of course it happened again a few months later on another calliper and the customer was not very happy.