Front brakes locked up

[frankachino]

A bit of background: A few months ago I was driving my 240 and the left front brake locked up on me. The caliper was completely seized and the pistons were not returning. I got the car home and rebuilt both front brakes with new piston seals and pads. After this the brakes were working flawlessly and I had no leaks. 

Fast forward to yesterday and the brakes locked up on me again. This time however, both front brakes were locked up and I noticed fluid leaking out of the top of the differential pressure switch. Left the car in a parking lot while I went to go get a trailer and by the time I got back to the car, the brakes were free and worked just fine. I tried replicating it in the garage, but no matter how many times I cycled the brakes, I couldn't get them to lock up again. I am planning on taking apart and cleaning the switch, but I am wondering if any one else has had this problem? I wouldn't think the switch would be the issue looking at how it works, but I can't think of what else could cause that. 

 

[Zed Head]

You might have the rod to the master cylinder adjusted too long.  Could also be the rod at the pedal to the booster.  This can close the fluid return hole to the MC reservoir.  The fluid expands and can't escape.

The fluid leaking out of the switch is probably coincidence or a result of the constant pressure on a small leak.

Anyway, there should be a small amount of play at the rod from the brake pedal.  Easy to check first.

[AK260]

That was exactly my first thought when I read the original post too!

[Zed Head]

It's been a while Mr. AK.  What's been happening?

[frankachino]

On 1/3/2023 at 12:23 AM, Zed Head said:

You might have the rod to the master cylinder adjusted too long.  Could also be the rod at the pedal to the booster.  This can close the fluid return hole to the MC reservoir.  The fluid expands and can't escape.

The fluid leaking out of the switch is probably coincidence or a result of the constant pressure on a small leak.

Anyway, there should be a small amount of play at the rod from the brake pedal.  Easy to check first.

I haven't had a chance to check the plunger on the master cylinder side yet, but the pedal side has a good amount of play in it before it starts actuating the booster. I should hopefully be able to take out the DP switch and check it to make sure there isn't anything gumming it up

[Zed Head]

Pretty easy to unbolt the MC from the Booster without removing the hydraulic lines.  Unbolt the warning switch so you get more flex in the lines.

The warning switch does not block flow like it is proposed to do some times.  It just moves the plunger to turn on the light.

There's a troubleshooting chart in the FSM.

[AK260]

It's been a while Mr. AK.  What's been happening?

Hey fella! I’ve been off the Z scene and all things Z related for just over a year. Just had way too much going on and training for the 2022 Etape du Tour took all my free weekend time. Poor Z only did 70-80 miles last year.

Great to be conversing with you and Z buddies again.

Apologies to to OP for my non thread related post. The brakes are pretty simple so I’m certain you will find the issues before long.

[frankachino]

3 hours ago, Zed Head said:

Pretty easy to unbolt the MC from the Booster without removing the hydraulic lines.  Unbolt the warning switch so you get more flex in the lines.

The warning switch does not block flow like it is proposed to do some times.  It just moves the plunger to turn on the light.

There's a troubleshooting chart in the FSM.

I took out the MC and turned the nut in on the booster. There was a good bit of preload on the MC from the booster so hopefully that will fix it. I did take apart the switch and it was absolutely disgusting inside. When I first rebuilt the front calipers I noticed there was a lot of gunk in there, so I'll probably have to take them apart again...

 

And no problemo AK! Always nice to meet new people!