Brakes slowly drag/lock/tighten

[Davidov]

This issue first occurred when I replaced my brake booster for my 1986 325. ABS.
A short while after replacing the booster(generic auto parts type), the brakes slowly dragged the car to a stop.
I cracked the R/R bleeder valve on the caliper loose to relieve pressure.
Then I bled the brake system thoroughly.
The problem happened several more times.

I then found the new brake booster included 2 shims. I removed the two shims, and the problem went away.
Now it's happened again, a year later. Same solution, bleed the pressure off, and it's good. (temporarily)

Now where do I start to look for issues?

[Davidov]

Read somewhere about adding a spacer between the booster and master cylinder.

[E30_fiend]

If you dont have the proper travel I'm the master cylinder push rod, you will either have no brakes, very little braking or you'll have dragging as you've described. The shim can be replaced but I feel a better action to take would be to adjust your adjusting rod on the brake pedal assembly. If there was a shim before behind the master, put it back. I have a feeling your adjusting nut may have loosened and that's why the problem appears to come and go.

Good luck.


Taylor

[Davidov]

Wouldn't the adjusting nut only make a difference for pedal height, and not engagement?

[E30_fiend]

Wouldn't the adjusting nut only make a difference for pedal height, and not engagement?

No, adjustingbthe pushrod will adjust the throw of the master. If it's out of adjustment then you won't get full braking power or none at all.


Taylor

[Davidov]

No, adjustingbthe pushrod will adjust the throw of the master. If it's out of adjustment then you won't get full braking power or none at all.

So it's possible to have just a little pre-tension on the brake pedal if it is not adjusted properly. Which could cause brake pressure to build up?

[AndrewBird]

If the clevis is adjusted too far out, it will slightly press on the booster as if you were slightly pressing the pedal. Over time, this slowly locks the brakes. There needs to be a slight amount of free play in the clevis with the brake pedal at it's normal resting position.

[Davidov]

If the clevis is adjusted too far out, it will slightly press on the booster as if you were slightly pressing the pedal. Over time, this slowly locks the brakes. There needs to be a slight amount of free play in the clevis with the brake pedal at it's normal resting position.

You mean "too far out" being out towards the driver's seat?

I adjusted the "U" clevis all the way towards the booster, and still had the issue. I'm having a hard time understanding how it affects the brake action.

The pedal is sprung upwards (towards the driver's seat), so there should not be any pressure on the brake booster pushrod.

[Davidov]

This problem never occured before the previous owner replaced the brake booster with the aftermarket (cardone) unit.

I'm going to try a used-OEM booster.

[LJ851]

The cause of your problem is that the bleed port in the master cylinder is not being uncovered because the piston is not being allowed back far enough.

The fluid in the system needs to be able to move back into the reservoir when it heats up and expands.

Allow the M/C bleed port to be open and your problem will go away.

[Davidov]

The cause of your problem is that the bleed port in the master cylinder is not being uncovered because the piston is not being allowed back far enough.

The fluid in the system needs to be able to move back into the reservoir when it heats up and expands.

Allow the M/C bleed port to be open and your problem will go away.

Agreed. I understand the basic principles behind this problem, but finding the practical solution is a different game.

The front pads are worn below min (brother's car, not mine ), and the front fluid lines have some external weathering/corroding.
1) New pads and rotors, front.
2) OEM booster
3) some SS brake lines (because they're cheaper than OEM)