34 replies to this topic

[mini_mad69]

1985 mini

non-servo master cylinder

metro vented 4 pot brakes freshly rebuilt with new pistons, seals etc

standard rear with mini fins

Adjustable bias valve in rear line.

New lines all the way through

One new rear wheel cylinder.

One almost new rear cylinder (100 miles)

I'm in the process of rebuilding my mini and can't get my brakes to work properly.

 

Pumping the brakes gets the pedal hard, which you can then hold and it will not go down. Leave the brakes alone for 30 seconds and the pedal pretty much goes 3/4 of the way down. Pressing hard goes to the floor, pumping it again brings it to the top.

 

I have bled around 4.5 litres of fluid through the whole system, and am pretty confident there isn't any air left.

 

So far I have plugged up the front takeoff from the master cylinder and the pedal is solid, ruling out the master cylinder at fault.

I took the callipers off and pushed the pistons in and out a few times and shook and tapped them while bleeding.

 

I then tried bypassing the t-piece for the offside calliper, effectively only having the N/s calliper on the front brake line, and I had a strong pedal with minimal travel and the brakes on that side working well. i reconnected the t-piece and I have the same problem. Pedal needs to be pumped and releasing it after 30 seconds or so means I have a huge travel again with minimal braking effot on the front brakes,

 

I am bleeding the system with my eazibleed and using a friend with the up-down procedure and it doesn't seem to work. 

 

Heeeeelp I've spent days on it and now narrowed down the fault to the o/s  caliper and t-piece.

There is no leaks in the system. I sat in the car with my foot hard down on the pedal for around 5 mins after pumping it and everything was dry.

 

The rear drums are adjusted correctly and work, the bottom part of the pedal works them fine.

[cal844]

Are the calipers on the right sides? What master cylinder is fitted? What brake fluid are you using?(DOT rating and type)

[mini_mad69]

DOT 4 brake fluid from motor factors. Calipers are the right way up. Can't remember master cylinder, its a front/rear split setup. Either green or yellow tag can't remember.

[Stu.]

You sure you have the rear drum adjusters correctly set?

[mini_mad69]

Yes, taking the o/s front caliper out of the equation gives a strong pedal. it must be something there.

[Stu.]

Ah, well at least you've isolated the culprit.

[mini_mad69]

Nothing seems to be really working, so took the caliper off. Blew it out with the compressor to clear all the fluid ways out. Took calliper off and the pistons are really really stiff in the cylinders. I mean, really really stiff.

I'm assuming this has something to do with it, also noticed, when I pushed the pistons out slowly with compressed air, they seemed to retract a little when I let off. I'm assuming this would be why pumping the brakes makes the pedal hard, and letting off the pistons may be slowly retracting?

[mini_mad69]

However, I've just gone out and pushed the pistons out with compressed air in order to get my calipers on them to make sure they are the right diameter, noticed one was alot easier, so pushed that one back in to seal them in order the get the others out and it went back in easily. The next one popped out and went in a bit easier. So clamped those and pushed the other two out. And again went in a little easier. So kept working them in and out and it got easier and easier. Now I can press them out with air and press them back in fairly easily by hand. So possibly these stiff calipers have been my issue and working them in and out may solve the issue? There was a little bit of grime that came out with the air as well, which ( long shot i know) could have been holding a bubble of air?

[dklawson]

A stuck piston should not be trapping any air and the pedal applied pressure to the system will be higher than what your air compressor can apply so it should be capable of moving the pistons anyway.  I know you said that the rear brakes were set up correctly and I know you said the calipers were installed correctly but please go back and check the following again.

 

Caliper bleed nipples pointed up (not down).

Rear brakes adjusted so that an elevated wheel can be spun by hand, you hear brake drag, and wheel rotation stops within about one revolution.

Since the shoes must have been removed to install the new slave cylinder, check the orientation of the shoe friction material.   

(Please see the recent thread linked below.  Pictures of the friction material appearance for the left side of the car are shown on page 2.  The right side will be mirrored to the pictures shown).

http://www.theminifo...shoes-dont-fit/

 

If the bleed nipples are pointed down, the calipers are on the wrong side (left/right) and they will trap air that cannot be bled out.

If the rear brake adjustment is loose and the wheels can free-spin, you will get excessive pedal travel on the first brake application and a higher firmer pedal if you immediately press the pedal again.  (This sounds like what you are describing).

If the friction material of the shoes is not oriented properly you can get a bit of excessive pedal travel but in general the pedal will just feel "soft" like there is still air in the system.

[Jordie]

check the hose(s) arent balooning under pressure

[mini_mad69]

Thanks for the advice guys, it is appreciated. I have braided hoses front and back and both seem in fine condition. Fitting the callipers back on after freeing up the pistons has made them slightly better. The pedal doesn't quite go to the floor, but still needs pumping in order to bring the pedal to a reasonable position for bite.

 

I will re-check the rear brakes for adjustment and do another bleed around the car. Starting to lose my mind with them now to be honest though.

 

Starting to think it may be the orientation of my brake lines. Forgot to mention I'm running a vauxhall engine with allspeed subframe, so the brake lines take a different route. Thinking maybe a high point in the lines could be causing an air trap, but don't think so.

[dklawson]

The low pedal on first pump and high, firm pedal on the second pump are more consistent with play in the system than air in the lines but it never hurts (except your wallet) to bleed the brakes again.  What brake bleeding method are you using?

[psychobob]

its worth bleeding the rear brakes the old fashion way. I had the same problem and it turned out to be the eazibleed. the regulator valve to the rear brakes was effectively shutting off the fluid to the rear as soon as it pressurised the system. I bled them with a oneway valved tube from the nipple then the other end submerged in a jar of fluid, so it could'nt suck any air back in. they are definitely better now.

[mini_mad69]

Fitted the standard mini 4 way splitter as advised by a brake guru who said this may help.

I don't think it did, I got the brakes useable but they bite was really low down the pedal without a pump. 

 

I just took the nearside caliper off, to work the pistons in and out and but like i did with the offside.

 

I now have no pedal at all until I pump them. Anyone got any other ideas? Getting really fed up with this car now not being able to use it.

[cal844]

Check the rear for adjustment!!