Brakes locking up and staying locked.

[gtmedley]

I am still battling a dragging / lockup up brake issue. After driving a mile or so, the brake pedal will be VERY firm and at the top and car slows down as all four brakes will not release. I have read all the posts on this. I have new brakes and MC. I have adjusted the booster piston four times now, turning in the little piston the goes into the MC.

I have now wound in the booster rod maybe 3-4 turns from the where I started. Feels okay for a couple miles of light braking -- no panic stops -- but ever so slowly the brakes start to drag. I bring my ratchet and extension with me and loosen up the MC mounting bolts a few turns, wiggle the MC. I tighten them back up and drive slowly home. The funny thing is, it did this about six months ago -- once -- and I turned in the booster rod one turn and the problem went away, until last week, and now it does it every time, even with me turning in the ros repeatedly. Strange.

According to the Haynes manual, the "spec" for the booster rod is for it to extend only 1mm past the face of the booster. Kinda touch to measure it, but I'm closes. But probably not exact.

All four wheels are affected. The only brake hose I did not replace was the center one that attaches to the rear braking compensator. Perhaps it is collapsing and holding pressure in the system. Or the booster itself is bad. What about the compensator valve is faulty, and affecting the entire system.

The car is a 1974 124 sedan, euro import. Any ideas would be appreciated. Thanks, Gary

[tima01864]

Next time you loosen the MC try a shim say 0.030 between the MC and the booster tighten that down, It may get you home and give an idea of the Booster rod. Anyone?

[Bills80]

Consider checking the flexible brake lines. I have heard stories of the inside of these breaking down and the pieces acting like a check valve. Pressure is applied but the loose material inside of the hose might go against the fitting and not allow the fluid to return. My ASE Master Tech son says he has seen this.

[DaveMarcotte]

If both front and rear are locking up it's a fault MC. I but a new one in and only a yr later it needed replacing.

[DieselSpider]

Some of the brake components for the Spider are not compatible with DOT4 or any kind of synthetically enhanced brake fluids so be wary of that also. The previous owner of mine was having all sorts of problem with his a few weeks after he had an emergency repair done at a tire store who put in DOT4 Full Synthetic and the solution was to replace the new hoses that had turned to gum inside from the DOT4 along with the new master cylinder which also showed compatibility issues with the DOT4 and then flush it all out and refill it with old fashioned DOT3.

There are still enough brake system components in the 124 Spider Parts Chain that have rubber components that don't tolerate anything other than old fashioned DOT3 very well.

[micbrody]

From one of my posts on 2016 after I replaced brake booster :
pushrod is adjusted too long.
I replaced my brake booster with the one from AR. I screwed down the nut and lock washer that came with unit all the way down. It did extend past the body of booster , but it looked to be within the 2mm that manual states.
When I started driving , everything felt great ; but within a few minutes , the braking became "super great"....I was realizing that my wheels were in constant braking condition. I theorized that initial driving was tolerable, but master cylinder must have been depressed slightly , trapping brake fluid in non-braking situation . As I continued to drive, the slight caliper pressure heated up brake fluid ; fluid expanded ; and more pressure placed on caliper.
I disconnected master cylinder from booster; pulled pushrod out. I removed the lock washer; and just screwed down rod with rounded end nut as short as it would go. Re-installed . Everything now works great