Spongy brakes, 914-4 brakes won't firm up??! Options

[Gearren]

I've had it! I have done everything to the brakes on my car, and I can't seem to get them to stop the car with authority. Best I get is a reluctant coast to a stop. Hardly suitable for street use! I have owned the 73 2.0 since new, and have kept it in the garage since 85. Brakes have always been weak, so as I starterd on the quest to revive the car, the brakes were top on the list. I have rebuilt both front and rear calipers, turned the rotors, new pads, new M/C (17mm), bled the m/c, bled the prop valve, and bled the system at least 10 times! I still have weak/spongy brakes! I want to keep the car stock, however I am tempted to remove the prop valve to see if it is the culprit. Any ideas or suggestions would be appreciated??

[davep]

The early stainless lines were not DOT. They tended to be oversize lines and can balloon a little under pressure and this can contribute to a soft feel. The prop valve reduces the pressure to the rear calipers and helps to keep the car from swapping ends. Depending on many factors including driver skill, this may be a good thing or not. For the factory, at the time, it was a good thing.

As much as anything else, I don't think the size of the master makes a big difference except that when the caliper piston sizes get too large, and require a large amount of fluid to move them the required distance, in which case it may require two pumps of the master to get effective stopping power. Two pumps of a small master to active big brakes is not considered acceptable due to the time required.

Manually setting the venting clearance on the rear calipers is the starting point. Once the car starts rolling, the wobble in the rotors will set the dynamic venting clearance. If the dynamic venting clearance is too large, then too much pedal travel is required to get pad engagement. The factory manual shows how to use a dial guage to measure disc runout, and they provide specifications on the allowable value.

Your pads may be glazed, or too hard of a compound to properly bite the rotor. I think your DOT 3 fluid should be okay. I do like Castrol LMA to reduce the water absorption and corrosion inside my system. There is a lot of factors at play and it takes a methodical approach to reach a solution. Even your tires may be too hard of a compound and don't grip well enough. Power assist brakes along with ABS make modern cars stop well, but it is hard to compare to a 35 year old design. Sometimes we just expect too much. Then we can justify spending money to create a system that meets expectations. (IMG:style_emoticons/default/beer.gif)