17 replies to this topic

[Coene614]

Hi, 

 

I recently rebuild the entire front subframe of my 1994 1.3 Mini: New brake hoses, pipes, rebuild calipers, new discs and pads, rebuild brake booster, new suspension bushings, etc.. everything was fine the first 200Km.

 

But then the brakes have started acting up, i would have a great initial pedal feel then suddenly the pedal drops down a little and you can feel it bite down again. 

 

It doesnt drop all the way to the floor and it keeps braking, but it feels weird that it has this "soft" spot you can push trough.

 

This then doesn't happen everytime you use the brakes so its always a suprise when it does.

 

Since this happend i bled the system multiple times, replaced master cylinder (green tag), rear wheel cylinders.

 

Something i have also noticed is that engine runs a little rougher when you come to a complete stop when the problem occurs, if you release the pedal it runs fine . 

 

When replacing the master cylinder i also found out that i switched the brake lines from the master cylinder to fam 7821 around. 

 

Afther switching these the problem became rarer but still occurs, could i have damaged the brake valve by plumbing it incorrectly maybe causing it to release pressure internally causing this issue?

 

Or does anybody ever had this problem, are these symptoms of a possible bad brake valve?

 

Thank you.

Edited by Coene614, 17 June 2024 - 08:21 AM.

[Spider]

Just so we are clear here, does it have a servo fitted ? And if it does, what type ?

[Coene614]

Just so we are clear here, does it have a servo fitted ? And if it does, what type ?

 

It has a servo fitted, GSM119 i used a rebuild kit for this one. 

 

Master cylinder is GMC90376

Edited by Coene614, 17 June 2024 - 08:20 AM.

[viz139]

Sounds like a leak on the vacumn side of the servo, will also allow air into the inlet manifold

[Coene614]

Sounds like a leak on the vacumn side of the servo, will also allow air into the inlet manifold

 

Any way to test this? I removed the master cylinder and when i pulled it of i could hear a hissing sound off the air being sucked into the vacuum 

Edited by Coene614, 17 June 2024 - 09:25 AM.

[viz139]

A vacuum gauge . measure the vacuum with and without the servo attached.  I think there may be no vacuum reserve in the servo when you press the brake pedal and than when the throttle closes the vacuum builds up and kicks in.  If you hadn't got the rough running I would think the non return valve but I'm more thinking a hole in diaphragm for air leak.

[Coene614]

A vacuum gauge . measure the vacuum with and without the servo attached.  I think there may be no vacuum reserve in the servo when you press the brake pedal and than when the throttle closes the vacuum builds up and kicks in.  If you hadn't got the rough running I would think the non return valve but I'm more thinking a hole in diaphragm for air leak.

Ok, will try to find a vacuum gauge this week and test it in the way you described and ill update when i know more. 

[coopertaz]

do you still have the non return valve fitted in the line from the servo to the inlet manifold. with engine rough running def. sounds like leak on servo diaphram. good suggestion with vac. gauge. vacuum Brake bleeding sets are good for this

[Coene614]

do you still have the non return valve fitted in the line from the servo to the inlet manifold. with engine rough running def. sounds like leak on servo diaphram. good suggestion with vac. gauge. vacuum Brake bleeding sets are good for this

Yes it's still fitted, this is the original valve with plastic line. thanks on the brake bleeder set tip will try this

[andyapanel]

I had a grommet fail where the vacuum line enters the servo.

[Coene614]

So i drove the car today and noticed that when i keep the brake pedal pushed in that the idle engine rpm begins to slowly rise from about 900 to 1300 rpm, it stays at 1300Rpm until you release the pedal. So i guess i'm indeed having a vacuum leak and that the engine is drawing in extra air via the servo thus increasing rpm?

[cal844]

Check the brake fluid level. Brake fluid can be pulled into the combustion chambers if the one way valve is faulty

[absx2]

Just a side note here but a while back a guy pitched up in a mini with a similar sounding problem that turned out to be a collapsed front bearing that caused the pedal to drop further but not all the time depending on how tilted the disc was.

I would say it was a one off but another came along that had a squeaking noise from the front wheel when braking, there was no noticeable play in the wheel when jacked up but on removing the hub bits of blue metal fell out.

 

Anyway more than likely a servo or the FAM7821 fault ( I hate those valves ) 

[Coene614]

So today i checked the servo with a vacuum gauge placed in line with a t connector, stationary its -0.6bar and when i fully press the brake pedal in the first half of the video its drops to -0.4bar for a second and goes back to -0.6bar.

In the second half of the video i’m pushing down gently like you would while driving normally the vacuum goes to -0.4 bar and engine rpm goes up a little you can hear it go up in the video, the vacuum stays at -0.4 until i press further or release the brake pedal.

Is this normal?

 

Also checked brake fluid level, ok , grommet on servo ok is a new one, 

 

Edit: Sorry no video...

Edited by Coene614, 20 June 2024 - 06:19 AM.

[viz139]

You may still need a manifold vacuum reading without the servo connected as a reference. We don't know if -0.6 bar is max vacuum or if there is a small leak.   

Simple video here of how a servo works and internal valves inside the servo that allow airflow at different points

https://www.google.c...bTUvp-tD5M,st:0