19 replies to this topic

[odl21]

weird question so please bear with me!

i've never managed to get the brakes on my mini so they actually work on the first pump of the pedal.  

 

i've recently rebuilt it with new rear brakes and it has cooper S discs on the front (it's a 1960 model so these were replaced).  i've bled them throughly and there is not air in the expelled fluid.  i've used both a vacuum pump and the two man method and put a whole bottle of fluid through to no avail.  they work great but only after a second pump of the pedal.  

 

the rear drums are adjusted so that i can get 3 clicks (may 4 at a push) on the handbrake.  the master cylinder is original type but it looks like the cooper S master cylinder is the same bore size (0.7") anyway with just a larger reservoir.

 

clearly i've bled brakes hundred of times before on other cars with no issues but no matter what i try the mini still has this issue.

 

what am i missing?

 

i'm wondering if perhaps the MC is actually a clutch MC with a narrower bore or something?

[sonikk4]

Hi and welcome to TMF. Have you fitted the front callipers on the correct side with bleed nipples at the top?? 

[hunterg30]

Have you checked the rear cylinders and adjusted the rears by the adjusting screws on the back plates . If so sounds like the master cylinder seals have gone

Edited by hunterg30, 06 August 2020 - 04:27 PM.

[GraemeC]

The brake and clutch master cylinders are easily identifiable by the relationship of the base plate to the can.
The can, outlet port and filler cap should be mirror images of each other with the base plates on the mounting studs.

 

I’d start by bleeding the master cylinder and chase the bleeding through from there.

Next I’d wedge the brake pedal down overnight.

 

If still no joy I’d suspect the master cylinder seals.

Edited by GraemeC, 06 August 2020 - 06:40 PM.

[MikeJE]

I hope you get an answer odl as I have the same problem. I, too, have discs on the front of my 62 mini but the calipers were seizing so I replaced them. Previously the pedal was tight, now it needs a pump just like yours. I, too, have tried pressure bleeding and the old fashioned method (which has always worked for me over many years). I have replaced first the rubbers in the master cylinder and then the master cylinder itself, but still the brakes need a pump to work properly. And yes, the nipples on the calipers are facing up. I have used some 3 litres of brake fluid through the system and can see no bubbles at all. Like you I am at a bit of a loss. So if you find the secret, please share it!

 

Good luck

 

Mike

[GraemeC]

If you don’t think it is master cylinder related then, if you have rubber hoses fitted, clamp a corner at a time and see if isolating one gives you a firm pedal.
If so, that‘s your suspect corner. If none make a difference then the problem is not at the wheels

[Spider]

When Bleeding the Rear Brakes, it's easy to have the Pressure Limiting Valve shut off, before you get a full 'bite' on them. You need to open the Bleed Nipple before pressing down the Pedal, so that the system doesn't develop any pressure in the Rears.

 

Also, if you have new or overhauled Calipers, the Seals maybe dry and so not allowing the Pistons to slip properly for self adjusting. To get these to slide properly, take one pad out at a time, pump the pedal a few times to move the Piston out about 1/2", press it back in and move on to the next until you've done all 4. This gets some brake fluid on the seals to lubricate them and allow them to self adjust properly.

[odl21]

thank you for all the sensible suggestions.  so going over them one at a time:

 

- the calipers are the right way round with bleed nipples at top.

- the caliper pistons are working smoothly. i even rebuilt one and found the banjo bolt in the passage from one side to the other had the cross hole drilled too far down and that was preventing bleeding properly.  that is now fixed but wasn't the problem.

- the MC is indeed correct (the clutch MC won't fit on the base plate correctly).   

- i have braided hoses so can't easily clamp off each corner without damaging the hoses unfortunately.

- the pressure limiting valve point is interesting, but i've tried vacuum bleeding them too and both methods provide plenty of fluid flowing out when the nipple is opened.

 

how do you bleed the MC?  there is no nipple on it.  what would the symptoms of leaking MC seals be?  i would think the second push of the pedal would slowly sink to the floor, no?  the pedal doesn't sink move with the second push.  

 

it seems as though the MC isn't pushing enough fluid for the calipers and slave cylinders.  were all MC's 0.7" bore with the same stroke?  

[GraemeC]

 

how do you bleed the MC?  there is no nipple on it.  

 

Just by cracking the unions

 

Also check the there is nothing stopping the pedal from returning fully, such as something under the end connected to the master cylinder

[Revd]

I’ve just overhauled the brakes on my 87, new m-c, new callipers, new wheel cylinders and new pressure limiting valve, these parts are not that expensive and replacing them eliminates an area of doubt. Using Gunston eezibleed open all four corners then shut them off starting with the back pair first and offside front last, I’ve been round three times. I’ve now got a firm pedal, currently touring Normandy with no issues.

[And again]

Hi just checking you adjusted the handbrake cable completely off before adjusting the rear drums then adjust cable for you’re 3 or 5 clicks

[Tornado99]

Not sure I fully understand the issue....is the pedal not firm for the first push of the day, then is good 'til next day? Is first push always soft, second and all subsequent presses then firm? Or are you only seeing first press just after bleeding is soft? 

The caliper seals on the mini rely on the rubber flexing as pistons move toward pads, then that rubber helps to pull pistons back by elasticity when fluid pressure is released. There is no slider pin arrangement on the caliper like modern cars so the caliper pistons are more difficult to get to the centered point....by multiple pumps of pedal so each piston can reach it's central point relative to pads on rotor. Once there, it should be good for next pedal action. 

 

One trick for doing a better bleed job, spread some grease around base of bleeder valve so that once open, less risk of introducing air past the threads if using vacuum or pressure bleeders. 

 

I've recently discovered gravity brake bleeding and really like it. Fit a hose on the bleeder, have fresh fluid in reservoir (suck out as much old stuff as you can easily with a syringe/baster/etc if renewing old fluid) and then open the bleeder 1/2 turn or so. Let the fluid flow out the tube (hopefully a clear one to see colour got lighter eventually) and into a catch bottle....got to keep the hose and bottle below level of reservoir so it can flow out under gravity alone. Drop by drop. Sure, takes a little while, but this is useful time where you can go to other wheels to prep them for their turn. Once tube/bottle shows new fluid, you are done, tighten bleeder, fill reservoir to appropriate level, clean up. Takes me about 10-15 min  per wheel. 

[MiniTim71]

Not sure I fully understand the issue....is the pedal not firm for the first push of the day, then is good 'til next day? Is first push always soft, second and all subsequent presses then firm? Or are you only seeing first press just after bleeding is soft? 

The caliper seals on the mini rely on the rubber flexing as pistons move toward pads, then that rubber helps to pull pistons back by elasticity when fluid pressure is released. There is no slider pin arrangement on the caliper like modern cars so the caliper pistons are more difficult to get to the centered point....by multiple pumps of pedal so each piston can reach it's central point relative to pads on rotor. Once there, it should be good for next pedal action. 

 

One trick for doing a better bleed job, spread some grease around base of bleeder valve so that once open, less risk of introducing air past the threads if using vacuum or pressure bleeders. 

 

I've recently discovered gravity brake bleeding and really like it. Fit a hose on the bleeder, have fresh fluid in reservoir (suck out as much old stuff as you can easily with a syringe/baster/etc if renewing old fluid) and then open the bleeder 1/2 turn or so. Let the fluid flow out the tube (hopefully a clear one to see colour got lighter eventually) and into a catch bottle....got to keep the hose and bottle below level of reservoir so it can flow out under gravity alone. Drop by drop. Sure, takes a little while, but this is useful time where you can go to other wheels to prep them for their turn. Once tube/bottle shows new fluid, you are done, tighten bleeder, fill reservoir to appropriate level, clean up. Takes me about 10-15 min  per wheel. 

 

Hi, 

" There is no slider pin arrangement on the caliper like modern cars so the caliper pistons are more difficult to get to the centered point....by multiple pumps of pedal so each piston can reach it's central point relative to pads on rotor."

 

I have just had a brake disk reduction fitted (local garage) on my '71 so that I can take it back to 10" wheels.  Brakes were working fine before (so MS etc should be good|).  But now brakes feel really soft, perhaps they need to bed in, but it feels more than that.  I am interested in this comment, so how do you find the center point of a caliper?

 

 

"One trick for doing a better bleed job, spread some grease around base of bleeder valve so that once open, less risk of introducing air past the threads if using vacuum or pressure bleeders.",       hmm interesting thought

[timmy850]

I had a similar issue with rebuilt calipers. When the car is at rest, check to see if the pads are loose in the caliper (just poke them and see if the rattle). If the pistons current resting position is too far in, it’ll take the first part of the pedal travel to make contact before you’ll get pressure

A similar concept to making sure the rear drum adjustment is correct - you want all pads and shoes within close proximity to the disc/drum so that all the fluid is used to clamp the brakes and not to push out the pistons a distance first

[Tornado99]

 

Not sure I fully understand the issue....is the pedal not firm for the first push of the day, then is good 'til next day? Is first push always soft, second and all subsequent presses then firm? Or are you only seeing first press just after bleeding is soft? 

The caliper seals on the mini rely on the rubber flexing as pistons move toward pads, then that rubber helps to pull pistons back by elasticity when fluid pressure is released. There is no slider pin arrangement on the caliper like modern cars so the caliper pistons are more difficult to get to the centered point....by multiple pumps of pedal so each piston can reach it's central point relative to pads on rotor. Once there, it should be good for next pedal action. 

 

One trick for doing a better bleed job, spread some grease around base of bleeder valve so that once open, less risk of introducing air past the threads if using vacuum or pressure bleeders. 

 

I've recently discovered gravity brake bleeding and really like it. Fit a hose on the bleeder, have fresh fluid in reservoir (suck out as much old stuff as you can easily with a syringe/baster/etc if renewing old fluid) and then open the bleeder 1/2 turn or so. Let the fluid flow out the tube (hopefully a clear one to see colour got lighter eventually) and into a catch bottle....got to keep the hose and bottle below level of reservoir so it can flow out under gravity alone. Drop by drop. Sure, takes a little while, but this is useful time where you can go to other wheels to prep them for their turn. Once tube/bottle shows new fluid, you are done, tighten bleeder, fill reservoir to appropriate level, clean up. Takes me about 10-15 min  per wheel. 

 

 I am interested in this comment, so how do you find the center point of a caliper?

 

 

You don't doing any thing to "find" the center position other than give the pedal a few hard pumps. The caliper pistons with push out until they both are in full contact with the pads against the disc/rotor If one piston reaches the pad at rotor before the other, fluid pressure will then force the other piston to catch up so both become even against the pads/rotor and both now get even pressure.