OT, but I need help diagnosing a brake problem please
 

the car: '72 bmw 2002tii.

new: brake calipers, master cylinder, clutch master, clutch slave, flexible brake lines, fluid...

i go to take the car out yesterday and after driving around for about 15 minutes, i start to lose brake pedal big time. pumping the brakes results in no change. so, i park the car and let it sit for a few minutes, and then, lo and behold the brakes are normal feeling again; nice and firm. so, i go take it for a ride, and once again, after about 10 minutes or so and the brakes are warm/hot again, i lose brake pedal. park it, and then they come back just like before. the brakes have been fine for about the last 500 miles after installing all these new components until yesterday.

i don't have any brake fluid loss, whatsoever. if there was air in the system, wouldn't the spongy pedal remain consistent during driving? this consistent cold/good, warm/bad cycle is strange to me. could my master cylinder have bitten the dust already??? and, if it did, wouldn't it die and stay dead, regardless of the temp of the brakes? i'm stumped. i'm going to re bleed the system tomorrow and see if the problem goes away, but i don't feel confident at this point. thanks in advance-

It appears you may have some very low boiling point fluid contaminating your brake fluid (like an alcohol). Did you flush or clean the hard lines with anything volatile? If so, an additional thorough flush with new brake fluid would be in order. Cheers, Jim

Been there before . . . try bleeding the brakes again. Start with RH rear, LH rear, RH front and finally LH front. Sounds like an air pocket in the lines.

jim, that's an interesting point. however, i've only run brake fluid through the lines. is it at all possible that different types of brake fluid could combine and cause similar behavior?

Well, the silicone based DOT5 types are not compatible with the glycol based DOT3 or 4 types but I'm not sure if they make a boiling point problem when put together. I think they make a corrosion problem by trapping water between the fluid types? Do you think you've mixed brake fluid types? Jim

i thought i flushed the whole system, but worst case scenario is that some dot 3 was mixed with dot 4... gonna go and flush and bleed again with dot 4 in about an hour...

I know you said you have new calipers..... but if one of the pistons is sticking or draging this will cause excessive heat and boil the fluid. Waiting 10 to 15 minutes lets everthing including the fluid cool off enough so you can drive a few more miles until it overheats again. I would check for sticky piston/caliper.

This is exactly the problem I am having. Cold (hard pedal) and warm (soft pedal). I have flushed with Super Blue and bled multiple times. At a recent AX, we noticed the right rear only smoke up upon hard braking (on a left hand turn no less). So, how do I debug this? If it is a caliper problem, how can I test this? And what would be a step by step approach to test master cylinder, calipers, lines, etc? I am assuming a pad check and full bleed is first. But after that, what to do and in what sequence.
Many thanks to the BBS in advance.
CL

Look also at your MC, could be a loose fill line, or bad seal

Okay. Will do.
Would I look for drips anywhere? Or wet? I certainly do not lose any brake fluid on the track or street.
How about the vacuum hose to the booster? From the various posts, it doesn't seem like that is often a problem.
Thanks.

Are any of the wheel hubs hot or hotter than the others when you have the soft pedal condition? If so, I would start there.
Are the pistons oriented correctly in the bores?
You might have gotten a 'bad' caliper.
Good luck.
Les