20 replies to this topic

[TheHunterHofman]

Hi all,

So after replacing my front brake discs, seals, pistons, pads and rings on my 2000 Mini MPI Sport, I note that the front brakes don’t work. I have bled the system with a pressure bleeder (started with the brake the furthest away from the master cylinder and worked my way up to the brake the closest thereto).

I note that the pads move ever so slightly but don’t make contact with the brake discs, and that the fluid in my reservoir comes up (and even overflowing a tiny bit), while pressing the brake pedal. I would say that this is an issue with air in the system, but don’t understand why, as I bled the system (more than once). What would explain the fluid coming up, is there really still air in the system? Is there perhaps an issue with the brake master cylinder?

Any input would be much appreciated.

Thank you in advance as always.

Regards,

TheHunterHofman

[Ethel]

The fluid reservoir is open to the brake lines until you push the pedal and the piston is pushed past the reservoir outlet. On a dual circuit master cylinder (under normal operation), the piston farthest from the pedal is pushed by the hydraulic pressure in the circuit nearest the pedal, so it needs cup seals at both ends. If the seal on the pushed end fails, the rear (i.e. nearest the pedal) piston could be able to push fluid back in to the reservoir via the outlet for the front circuit.  

 

The rear piston has an extension  that will make mechanical contact with the front piston, in the event of pressure loss in the rear circuit, so the front circuit continues to operate.

 

If you're having persistent problems with hydraulics, you could make up some  stops for isolating parts of the circuit to eliminate, and so locate, where the problem is.  A brake union with a short length of pipe crimped over a few times screwed in, in place of the rear line on the master would confirm if your symptoms are from within the master, by eliminating any possible external causes.

[Mr Piggy]

As Ethel says. The fluid in the master cyl bore is being displaced by the piston and pushing it back into the reservoir. You have a seal faiure in your master cyl if this is happening.

[nicklouse]

Seals have gone in the MC.

[TheHunterHofman]

This is very useful information, thanks all. Will follow-up when fixed.

[TheHunterHofman]

Apologies for revisiting, but I just installed a brand new master cylinder (the AP Lockheed one) and the problem still persists. It’s driving me up the wall. So I noted some brake fluid coming up the reservoir the first time, when pressing the brake pedal with the engine off (after of course bleeding the brakes). When I started the engine and pressed the brakes, I could then feel some resistance when pressing the brakes hard several times, only to lose brake pressure again after letting it pause for 3 to 5 seconds, then the pedal went soft again. Then, when pumping the brakes hard several times again, it builds up resistance again, only to lose that resistance again after letting it rest for a second.

What could this issue be? Does it have anything to do with a vacuum line? What about the brake servo? Electronics? Happy to hear your thoughts.

Thanks in advance, all.

[TheHunterHofman]

Apologies for revisiting, but I just installed a brand new master cylinder (the AP Lockheed one) and the problem still persists. It’s driving me up the wall. So I noted some brake fluid coming up the reservoir the first time, when pressing the brake pedal with the engine off (after of course bleeding the brakes). When I started the engine and pressed the brakes, I could then feel some resistance when pressing the brakes hard several times, only to lose brake pressure again after letting it pause for 3 to 5 seconds, then the pedal went soft again. Then, when pumping the brakes hard several times again, it builds up resistance again, only to lose that resistance again after letting it rest for a second.

What could this issue be? Does it have anything to do with a vacuum line? What about the brake servo? Electronics? Happy to hear your thoughts.

Thanks in advance, all.

[TheHunterHofman]

Apologies for revisiting, but I just installed a brand new master cylinder (the AP Lockheed one) and the problem still persists. It’s driving me up the wall. So I noted some brake fluid coming up the reservoir the first time, when pressing the brake pedal with the engine off (after of course bleeding the brakes). When I started the engine and pressed the brakes, I could then feel some resistance when pressing the brakes hard several times, only to lose brake pressure again after letting it pause for 3 to 5 seconds, then the pedal went soft again. Then, when pumping the brakes hard several times again, it builds up resistance again, only to lose that resistance again after letting it rest for a second.

What could this issue be? Does it have anything to do with a vacuum line? What about the brake servo? Electronics? Happy to hear your thoughts.

Thanks in advance, all.

[cal844]

Where are the front caliper bleed nipples in relation to the flexi hose (nipple needs to be at the top)

Another tip is to clamp off each flexi hose, if the pedal improves once clamped you've found the problem corner.

I've also had to 'chase' the air along lines, bleeding at each union until fluid weeps out, nipping back up a little tighter than before and going to the next junction.

There is no electronics on the MPI brake system

[Ethel]

I didn't pick up on it being a late model with a horizontal servo 

 

 

I suppose that could cause problems, but only if the servo was holding some vacuum. If the engine isn't run, or the hose isn't connected, it would lose that in a few operations. 

 

The only other place you'd be able to squirt fluid from one circuit in to the other is the FAM7821 regulator valve. Doesn't seem very likely to me.

[lildeucecoop72]

If a dual system did you pre-bleed /bench-bleed the new MS before applying it to the system?

[TheHunterHofman]

If a dual system did you pre-bleed /bench-bleed the new MS before applying it to the system?

I did not, no. Also, after removing my ‘old’ master cylinder, I have not pre-bled it after reinstalling it. Is that strictly necessary to do? Don’t you bleed out any air via the usual bleeding of the brake lines? I wasn’t aware of pre-bleeding a master cylinder

[TheHunterHofman]

I didn't pick up on it being a late model with a horizontal servo


I suppose that could cause problems, but only if the servo was holding some vacuum. If the engine isn't run, or the hose isn't connected, it would lose that in a few operations.

The only other place you'd be able to squirt fluid from one circuit in to the other is the FAM7821 regulator valve. Doesn't seem very likely to me.

So is there any troubleshooting I can do in regards of the servo holding vacuum?

I’ll look into the FAM7821 regulator valve.

[lildeucecoop72]

Check out this vid on pre bleed or bench bleeding here: 

 

 

EDIT: ...though I will admit I am not sure if it is necessary on a servo'd MS.

 

Best check with someone who knows (anyone?) before you spend lots of time on it. :)

 

 

 

If a dual system did you pre-bleed /bench-bleed the new MS before applying it to the system?

I did not, no. Also, after removing my ‘old’ master cylinder, I have not pre-bled it after reinstalling it. Is that strictly necessary to do? Don’t you bleed out any air via the usual bleeding of the brake lines? I wasn’t aware of pre-bleeding a master cylinder

 

 

 

Edited by lildeucecoop72, 05 February 2023 - 05:39 PM.

[Mr Piggy]

It sounds like you have a stubborn air lock. Without the pressure bleeder connected, open a front bleed nipple and stamp on the brake pedal. If you haven't got a helper, release the pedal very slowly, if you have a helper, close the nipple for the up stroke. Do this a few times. Then bleed as normal.