12 replies to this topic

[AJTMini25]

Hi guys, I recently changed the rear shoes and slave cylinders along with the nearside front caliper on my 1984 Mini 25. My dad and I bled the brakes multiple times and cleared all the old fluid out at the same time. I'm 100% sure there's no air in the lines anymore.

 

It has a yellow tag master cylinder and front-to-rear split with NO SERVO. My brakes never used to be amazing (what do you expect though?), but now I don't get much braking force at all. There's enough to stop me but I don't feel totally save (neither do my passengers). 

 

If anyone has any suggestions I would be really grateful! 

 

Cheers, Andy

[Carlos W]

Rear brakes properly adjusted?

[AJTMini25]

Yeah we went through it all thoroughly, there's no leaks in the lines either

[Carlos W]

What size rear wheel cylinders did you fit?

 

Was the caliper new or refurbished?

[AJTMini25]

They're TRW ones from motor parts direct. Not sure on size but they matched the old ones. 

 

The caliper was brand new from Somerford Mini Spares. 8.4"

[AJTMini25]

Any other ideas guys?

[Dan]

Are you sure the rear cylinders matched what was removed? They pretty much all look similar but have different bore sizes, this affects the hydraulic balance of the system. Do they have a part number or a bore size stamped on them?

[AJTMini25]

The part number is BWD194. Not sure about the bore diameter, I believe it's 19mm?

[tiger99]

It will not be 19mm, I think, but it could be 3/4" (19.05mm), which is very close. But even if they were making replacement cylinders in metric dimensions now, it would be close enough to not have any tangible effect.

 

When you apply the brakes HARD, does the pedal stop well off the floor, or is it bottoming out before you get full braking. If it stops well clear of the floor, your hydraulics can be more or less eliminated as the cause of the problem, and it is down to the friction surfaces themselves. If it goes right down, there is still air, or the pipes and hoses are flexing under pressure. (You didn't use copper,I hope? Cunifer will be fine, or steel, and almost everyone agrees that Goodridge flexibles are best.) Or, just maybe, the calipers are spreading under pressure. (My old Triumph Herald had very sharp brakes, discs at front and no servo, but if you pressed VERY hard, way beyond the point of wheel locking, a level of force that only a strong person would achieve, you could force the pedal to the floor, and it was due to elastic deformation of the calipers and maybe the rear drums too. I only found that out when testing the overhauled hydraulic system for leaks, under extreme pressure.)

 

None of this "should" be a problem of course, using parts that were intended to work together in a standard Mini configuration. The calipers should not be spreading, for example, but I am just putting those thoughts forward because you seem to have checked all the obvious things already.

Edited by tiger99, 26 March 2014 - 01:53 PM.

[AJTMini25]

Thanks for the suggestions.

There's no real braking force until you get right to the bottom of the pedal's travel. I rung a specialist earlier today and they were saying i need to try bleeding a few other places but ive forgotten what they were. If not then sounded like maybe a master cylinder failure and might only be operating front or rear.

[tiger99]

Well, if the pedal is going down a long way, it could be a failure of one brake circuit, and if you are not losing fluid it can only be internal to the master cylinder, i.e. the fluid is going back into the reservoir instead of the brake cylinders due to seal failure. But try bleeding one more time, with the rear brakes adjusted up TIGHTER THAN TIGHT, for want of a better way of describing it (handbrake off, it is only the adjusters that want to be tight). That way the rear pistons are fully retracted, so any trapped air there is minimised.

 

Check that calipers are right way up, bleed nipples at TOP. I expect you have already done that, but it does come up regularly here.

[AJTMini25]

I'll do that at some point this weekend, thank you. I was brake testing today on the way home and the front locked up (when pressed flat down and quickly) or the front end dived as it should but didn't seem to be getting much effort from the rear. This all sounds like there's a master cylinder issue so I'll check that out too

[dklawson]

The shoes on the rear are new.  Were you aware during installation of the new shoes that there is a proper oriention of the friction material relative to the direction of wheel rotation?  I am going to try and post a picture from Mini Sport since Mini Spares pictures don't always link for me.

 

Look at the bottom assembly in the foreground.  That is a LEFT hand rear brake backplate assembly.  Look at the friction material on the shoes.  There is part of the shoe where the friction material is removed.  That relieved area is positioned so it would contact the inside of the drum FIRST as the wheel rotates.  The RIGHT side assembly will be the mirror image of the left.  Take a look at your brake shoes and make sure they match the picture.

 

Once you are confident the shoes are on correctly, adjust the brakes up very tight (like the wheels won't turn tight) and re-bleed the brakes.  Then readjust the drums all around.  Proper adjustment is NOT a free spinning wheel.  If you elevate the wheel and turn it by hand you should hear and feel a bit of drag on the shoes.  On the rears, you should be able to spin the wheel and when you let go it stops within about one revolution (or less). 

 

Try those two things and see if your pedal is not both higher and firmer.