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Brakes - Bleeding Problem

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#1 oldmanmini

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Posted 10 June 2020 - 03:25 PM

Hi all.

 

I have a G registered (1990) Mini fitted with calipers at the front and servo assisted as standard on this model.  I have just replaced both front calipers and pads (brand new), along with all flexy's and most of the pipe work. (Under the bonnet not changed - but good) Rear pads adjusted. And a new brake master cylinder also fitted. All parts from a reputable Mini dealership in the UK.

 

Prior to the work the brakes had a very good biting point - but had to change as one caliper was badly sticking and some pipes corroded.

 

The issue I have is that: The brakes have been bled and no air appears.  But when driving the rear brakes have a much greater efficiency than the front; however when I pump the brake pedal a few times immediately before applying the brakes the fronts work as they should and the whole brake system appears as it should with the car stopping nicely and in a straight line.  The brakes revert to their rear wheel first version after a few minutes when brake pedal released. 

 

There still appears to be some air in the system, but despite further bleeding I cannot seem to remove it - the brakes have been bled using the two person method as well as Gunsons single bleed system.  Whole system visually checked, I cannot see any leaks, and the brake fluid level is not going down in the master cylinder reservoir. 

 

Has anyone got any ideas.  


Edited by oldmanmini, 10 June 2020 - 03:35 PM.


#2 GraemeC

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Posted 10 June 2020 - 03:47 PM

Usual first question - are the calipers on the correct sides with the bleed nipples at the top?



#3 oldmanmini

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Posted 10 June 2020 - 03:50 PM

Yep.  The bleed nipples are on top.  



#4 cal844

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Posted 10 June 2020 - 04:10 PM

Try opening the nipple with the pressure on the brake pedal, close it with the pedal down on the floor, repeat 2 or 3 times and move on to the next corner.

#5 Chris1275gt

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Posted 10 June 2020 - 05:46 PM

Had the same problem last year the front pads were being pulled back off the discs by the new rubbers being dry. After taking Moke spiders advice I removed 1 pad at a time then using the pedal push the piston out about 10mm ( don't let it come too far) then very sparingly applied a bit of brake fluid to the piston then levered it back in and thoroughly cleaned up any fluid. Replace the pad and move on to the other 3 pistons. From being pumped twice to firm the pedal I had a normal pedal.

#6 Ado1379

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Posted 10 June 2020 - 06:58 PM

I also had a similar problem on my old SPI (could be similar?) where the brake limiting valve underneath the servo was weaping. It wasn’t a big enough leak to leave a puddle under the car but always introduced air in the system. Might be worth checking

#7 oldmanmini

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Posted 11 June 2020 - 02:51 PM

Many thanks for the piston solution and the brake limiter.  I'll look at them both - interesting - Thanks 



#8 cal844

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Posted 11 June 2020 - 05:58 PM

Many thanks for the piston solution and the brake limiter. I'll look at them both - interesting - Thanks


The brake valve on the bulkhead (FAMwtf valve) is non serviceable so I'd just try loosening the tube nut on the output side to try and bleed it. Loosen just enough until the fluid weeps around the pipe then tighten fully

#9 weef

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Posted 11 June 2020 - 10:26 PM

Hi  first time bleeding from an almost empty system can sometimes be a bit challanging. Even when using a pressure bleed tool it still can leave air in the system for all there appears to be no air trapped . I have found that it usually is the front system the air is trapped, and I think once the rear system is purged it restricts the pedal movement so a good purge pump is not achieved on the front system. In the past I have bled the system by opening both a front and rear bleed valve together and have an assistant pump the pedal. Once the pedal is down, hold, close both valves, and release the pedal. Carry on with this method, keeping the master cylinder topped up, for several pumps then move on to the next pair of bleed valves. You might have to repeat this method a couple of times on each pair until a good pedal can be achieved. 

I think when there is air trapped in the system it just "see saws" back and forth in the pipe with each pump and never manages to get clear.

As I say I have experienced this problem a few times on systems that have had a lot of work done and the system is either empty or nearly so.

The first time you get it you wonder what you have done wrong or if some component fitted is defective but usually it is found to be air trapped somewhere.

Its just a right pain.



#10 Quinlan minor

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Posted 12 June 2020 - 09:32 AM

As weef says, above, it can be a challenge.

 

I have found, as long as the brakes are just 'long pedal spongy' working, it can help to take a little test drive on bumpy roads (not difficult to find nowadays) before re-bleeding.

 

Be careful to follow the order of bleeding to avoid problems on the dual circuit models. Haynes is useful on this.



#11 oldmanmini

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Posted 12 June 2020 - 01:25 PM

Hi all.   Live in a rural setting so went on a bumpy road ride, carefully of course.  Re bled the brakes and did find a little 'air bubbles' from the front system. Also removed some of the 'free' movement that was in the pedal to servo linkage.  Re tested and MUCH better.  The brakes feel firmer with the normal bias to the front.  Thank you all for your response.  



#12 sonscar

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Posted 12 June 2020 - 07:24 PM

Make sure there is enough free play in the servo rod as the brakes can lock on if insufficient.Steve..





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