16 replies to this topic

[NLinPEN]

Recently I removed the brake master cylinder from my car to fix a fluid leak between the reservoir and the cylinder. More about that can be found here: https://www.theminif...itment-problem/

 

After installing the master back in the car I am now having problems in bleeding the brakes. There is not much pressure on the brake pedal: I am able to push it to the floor. When bleeding the brakes I notice that fluid does come from the bleeders at the drums in the back. However, there is no fluid coming from the bleeders at the calipers in the front. Nothing, not a drop. Neither from the left, nor from the right hand side brake. Also, when undoing the bleeder there is no change in feeling on the brake pedal. When undoing one of the rear bleeders you can feel the brake pedal having less pressure. Not so with any of the front bleeders. I have tried bleeding the brakes with both engine off and engine on: makes no difference.

 

I am wondering what might be wrong, causing this to happen? Any hints or tips on how to proceed with this?

[sonscar]

Have you blocked the outlet from the reservoir,got an airlock?Open the pipe fitting that feeds the fronts at the master cylinder.Steve..

[Spider]

If you've used a rubber grease when assembling the Master Cylinder, this can block the very small transfer ports between the Reservoir and the Master Body. This is where I find a Bleed tool like the Gunsons particularly handy to get the fluid started. Once it starts to flow and wets up the inner of the Cylinder, it'll go like smoke.

[weef]

You will need to determine where the fault lies before you go any further.

Firstly, with the master cylinder resevoir full remove the two pipes that go to the braking system, with the brake pedal in the normal rest possition fluid should run from both ports if all is well with the master cylinder, if not check the master cylinder.

You can fit blanks or bleed nipples to the master cylinder, bleed it and check it works, i.e. the brake pedal should be pretty solid.

Once you have determined the master cylinder is OK connect up the car pipework to it and bleed the system.

You have given yourself a lot of grief by opening the bleed nipples at the wheels , you would have got away with bleeding the system at the first pipe union from the master cylinder.

Bleeding the brakes, in theory, is a straight forward procedure, but on thr Mini can prove tiresome and time consuming.

As suggested the use of a brake system bleeding tool, either a pressure or a vacumm unit is a big help.

[NLinPEN]

Thanks all for your responses and suggestions. I had put this topic to rest for a day before returning to the car. I wanted to undo the bleed nipple to make sure that it wasn't blocked/clogged. To my surprise did fluid start to flow out of it. In other words: the brake pipes had filled themselves with fluid overnight, to both calipers. I guess that gravity did its work. It had been my intention to put the brake bleeder tool to work: it's a tool whereby you put pressure on the fluid reservoir, thus forcing fluid into the master cylinder. This makes it possible to bleed the brakes without needing the help of an assistant. The tool helped in the sense that I get good fluid flow at all four bleed nipples. But in the end the brake pedal still doesn't feel to my liking. With the engine off it feels hard, as desired. But with the engine on (and thus vacuum helping the brake booster) the pedal is very soft and easily pushed to the floor. I need to do further digging to find out what's going on here. Previously I had similar issues (hard pedal with engine off, soft with engine on), but I forgot how I solved it at that time.

Edited by NLinPEN, 26 August 2026 - 02:51 AM.

[weef]

I'd try bleeding the brakes using the tool you have to see if there is any improvement, there may still be some air in the system.

Allow a decent ammount of fluid to bleed out at each wheel.

[NLinPEN]

I'd try bleeding the brakes using the tool you have to see if there is any improvement, there may still be some air in the system.

Allow a decent ammount of fluid to bleed out at each wheel.

Indeed, that's what I did today: bleed one more time, using the tool that puts pressure on the brake fluid. And let plenty of fluid flow before closing the bleeder nipple (repeat for each wheel). There wasn't even the tiniest air bubble visible any more. Unfortunately it didn't help: the feel of the brake pedal was the same after as it was before.

One thing that I did notice is that when I push the brake pedal I hear a soft "plop" sound when I start pushing the pedal down. And hear the same sound on the way up, shortly before the pedal has been released completely. This makes me worry whether a seal in the master cylinder is broken. Another, but less likely, hypothesis could be that there is a problem with the diaphragm in the brake booster. I think this one is less likely as there clearly is a different pedal feel when the engine is running (i.e. vacuum present) than when the engine is off. The plop sound is audible in both cases.

Anyway, I won't be able to work on the car in the coming weeks. But think that my first action will be to pull the master cylinder, disassemble it, and inspect all seals. I will already buy a seal set (GRK1032, at Minispares) in advance, as to not lose more time once I'm able to work again on the car.

Edited by NLinPEN, 28 August 2026 - 09:25 AM.

[weef]

Before you strip it remove the pipes from the master cylinder, blank off the master cylinder ports to see if you have the same symptoms to try and identify if indeed the fault is with the master cylinder / brake servo.

[bpirie1000]

The 'PLOP' You mention would,d put me towards a sticking pedal to be honest.. is the pedal retaining shaft in the pedal box free and lubricated?

The servo type also has 2 mechanical elbows that could be causing issues...

Might be worth a look see.

[croc7]

Recently I removed the brake master cylinder from my car to fix a fluid leak between the reservoir and the cylinder. More about that can be found here: https://www.theminif...itment-problem/

 

After installing the master back in the car I am now having problems in bleeding the brakes. There is not much pressure on the brake pedal: I am able to push it to the floor. When bleeding the brakes I notice that fluid does come from the bleeders at the drums in the back. However, there is no fluid coming from the bleeders at the calipers in the front. Nothing, not a drop. Neither from the left, nor from the right hand side brake. Also, when undoing the bleeder there is no change in feeling on the brake pedal. When undoing one of the rear bleeders you can feel the brake pedal having less pressure. Not so with any of the front bleeders. I have tried bleeding the brakes with both engine off and engine on: makes no difference.

 

I am wondering what might be wrong, causing this to happen? Any hints or tips on how to proceed with this?

Did you bench bleed the master cylinder prior to installation?

[NLinPEN]

 

Did you bench bleed the master cylinder prior to installation?

 

No. The reason is that the thread for the two brake pipes don't go very smoothly into the master cylinder. When I'm holding the master cylinder in my hand I can thread any one pipe easily. But the pipes cause tension on those flare nuts when I want to install the brake lines with the master (loosely) in position. Each time it takes me quite some jiggery pokery to get both threads going and not cross-thread it. All brake fluid would have dripped out of the master cylinder by the time I would have those threads going. So it would be useless to pre-fill the master with brake fluid, only cause a mess in the engine bay.

[NLinPEN]

The 'PLOP' You mention would,d put me towards a sticking pedal to be honest.. is the pedal retaining shaft in the pedal box free and lubricated?

The servo type also has 2 mechanical elbows that could be causing issues...

Might be worth a look see.

Good suggestion: I haven't inspected those points. I will try that once I have removed the master cylinder.

[NLinPEN]

Before you strip it remove the pipes from the master cylinder, blank off the master cylinder ports to see if you have the same symptoms to try and identify if indeed the fault is with the master cylinder / brake servo.

Newbie question: how do I blank off those two ports? I guess there can be quite some hydraulic pressure on those points.

[Quinlan minor]

No. The reason is that the thread for the two brake pipes don't go very smoothly into the master cylinder. When I'm holding the master cylinder in my hand I can thread any one pipe easily. But the pipes cause tension on those flare nuts when I want to install the brake lines with the master (loosely) in position. Each time it takes me quite some jiggery pokery to get both threads going and not cross-thread it. All brake fluid would have dripped out of the master cylinder by the time I would have those threads going. So it would be useless to pre-fill the master with brake fluid, only cause a mess in the engine bay.

 

Try a double sheet of cling film under the reservoir cap. That should stop the pre-fill flowing out while you connect the pipes.

[weef]

To stop the brake fluid leaking from the master cylinder resovoir when the master cylinder pipes are disconnected, press the brake pedal to about mid travel and lock/ hold it in this possition. This action closes the resovoir feed to the master cylinder ports so none leaks from the pipe ports.

This applies to all master cylinders no matter the application.