Brake bleeding

[Downton1960]

Hi everyone

I am having a nightmare bleeding the brakes on my Mk1

It is running;
7.5” Cooper S discs, Calipers
New good ridge hoses
Cooper S master cylinder
Cooper S brake regulating valve
After market remote servo 1.90
Mini fins
New rear AP cylinders
New brake switch and all new brake lines.

I have spent a couple days bleeding the system and keep getting the same issue that you can get a good pedal but wait 5 seconds and it drops to the floor, pump it twice and it’s firm again.
Today I rebuilt the master in case it was defective but no change, I have been around the system and have no leaks.
Yes my front calipers bleed nipples are on the top.
I have tried bleeding furthest away working to drivers side and also tried what my manual stated bleeding sided.
I have bleed countless systems in the past with no issues?

Unfortunately my easy bleed was leant to a neighbour a long time ago and I can’t collect due to lockdown so doing a traditional two person bleed.
The only cheap part I have used is the brake servo which was new but came from eBay so could this account for my symptoms?

I have bleed at the servo fittings and get no air and tried jacking the car up both ways with no change.
I have gone through 1.5 litres of fluid trying to sort this so really am at wits end now

Any help or advice would be appreciated
Dave

[Herbert]

Its definitely your master cylinder. Was it a new one or did you repair it? Get a new one and try again. Sorry. R

[Downton1960]

Hi R
Thanks for the reply
It was a brand new Cooper S master a few months ago, this is why I thought it can’t be leaking. I ordered a repair kit from mini spares and rebuilt it today with exactly the same issue? It was perfect inside as expected being new, the seals looked good as also but still changed them out.
I’m still suspecting the remote servo but don’t know if that gives these symptoms?
If the view is the master then I will buy a new one but never had an issue in the past with these from minispares.
Dave:)

[Downton1960]

Forgot to mention I have checked the rear brake adjustment and all good.

[dklawson]

I do not suspect the master cylinder. I suggest the following.

Adjust the rear brakes. I know you said the rear brake adjustment is good but check it again. Proper adjustment is when an elevated rear wheel can be spun by hand, rotates no more than one turn then stop, and you can hear a bit of drag. Being able to spin the wheel and have it coast through multiple revs is too loose. When the rear brakes are out of adjustment you can typically pump the two or more times and get a firm pedal and the pedal will stay firm until you release it and wait a few minutes.

Make sure that when the master is at rest that the pushrod is not displacing the piston. I have not experienced this but I have been told it causes a number of brake problems. (I hate passing on hearsay but this is an easy thing to check).

Then consider the following dry system bleed method.
Crack open the joint where the first pipe screws into the master cylinder. Gently press the brake pedal while an assistant watches. When fluid starts dripping out of the joint, have your assistant tighten that pipe joint.
Move down the brake line to the next fitting. As above, crack the joint open and gently depress the brake pedal. When fluid is seen dripping out, tighten that joint.
Keep moving down each line to each wheel cylinder/caliper opening each joint and pumping a bit of fluid out.
When you are ready to work on the rear cylinders depress the pedal VERY slowly and gently. If you depress the pedal rapidly and with a lot of force you can/will shuttle the spool in the rear mounted proportioning valve. Once that shifts you cannot get fluid (or air) out of the rear brake lines.

You could also try gravity bleeding one line at a time instead of the active method I propose above. Some have had success with that. But check the rear brake adjustment first.

[Spider]

One other thing to check is the front calipers.

If they are new or recently overhauled, the seals maybe dry, not allowing the pistons to self adjust.

Easy fix. Take one pad out, pump the pedal about 5 - 6 times, to get that piston to extend out about 10 mm, push it back, refit the pad. Do the same with the other 3. This will get some fluid on the seals and lubricate them.

Another thing to check on the Servo. Is the outlet Pipe higher then the Inlet Pipe? If not, it will hold air.

[Polarsilver]

Remember there was a lot of Duff seals supplied in new S master cylinders sometime last year + if it was me & i suspected that the Ebay Servo could be an issue why not remove the Servo & pipe up your brakes as a none servo system & without that servo fitted,does that then fix your pedal feel .. but before you take the Servo off check if you have it piped up the correct way around ?
If not already checked out i would also have a look inside the rear brake balance valve .. horrid things that are difficult to evaluate.
Last thought make sure you keep enough Brake Fluid in the Master Cyl while bleeding .. when my assistant pumps i count the 10 pumps down & then re=top the brake fluid just to be sure i do not draw in air to the system.

Otherwise i do agree with the other thoughts & comments on this post.

[Downton1960]

Thanks everyone

I will try to cover everything

I didn’t know about the bad Cooper S master cylinders, tbh when I took it apart today it was perfect and rebuilt quite a few of these which worked fine but in a terrible state inside. I almost just put it back together but thought better so changed the seals anyway. I had nil leakage into the car on either occasion.

I will make a pipe tomorrow and bypass the servo just to confirm it either way along with the master.

I did bleed through some connections like both of the servo and the rear brake regulating valve but not every union on the car, I did check every Union for leaks on several occasions.

I was not aware about the slow pedal for rear brake bleeding as my wife is quite heavy footed

Pretty confident with the rear wheel adjustment though as that’s how I alway set them (Owned my 1972 mini since 1991 although desperate for a restoration now and was next on the list). I will double check them and make sure the rear cylinders aren’t leaking tomorrow as well although again new AP items.

The front calipers and pads are brand new from mini sport but I will try the pad trick as need to adjust the ride height anyway.

The servo outlet is higher and set up as per several articles I have read on here and online so should be good hopefully.

Out of curiosity what would a duff servo do? I had one fail years ago where it leaked into the vacuum chamber but not losing any fluid?

I will look at the pushrod but when I removed the cylinder today and put it back it slid over the pedal with ease and whole job with pin removal only took around 30 mins including rebuild.

Fingers crossed for day 3, getting low on fluid and never reuse if anyone asks.

Appreciate all the thoughts and gives me something to go on, I will update progress

Dave

[OGW 1082]

Hi Dave,
Lots of good advice there. The only thing I'll add is if you have an air compressor
a vacuum bleeder works really well. I use one on my cars to change the fluid.
Hook up to the compressor and bleed nipple and pull the trigger. One person operation and
avoids the pressure regulator issues, not to mention finding someone to pump the pedal.
Glen

[Aussie Bill B]

You could make a simple pressure bleeder adaptor with an old (or new) reservoir cap.
Remove the air filter part of the cap; drill a suitable hole through the cap a Schrader fitting for an alloy wheel.

Master cylinder Mod 1.jpg

Fill the reservoir with fluid. Fit the cap.
Use a locking chuck from a compressor on low pressure setting;
OR just use a bicycle pump to pressurise the master cylinder.
Bleed the rear LH wheel cylinder until clear fluid.
Alternate pumping and topping up master cylinder.
Work your way around the car. Don't need to mess with the booster and eliminates sucking in air bubbles.
Minis seem to get better pedal pressure once they are being used.

[JohnA]

Maybe a bit obvious, have you fitted the calipers the right way round
I had a customer with an MGB that couldn't get a decent pedal and when he brought the car in he'd fitted them to the wrong sides with the bleed nipple at the bottom

[imack]

Just a few things to add to the above suggestions.
I do as Spider advises and pump the pistons in and out but when the piston is exposed I put a drop or two of brake fluid on the piston and then work it in and out.
I also bleed troublesome brakes with caliper pistons and wheel cylinder pistons pushed right in or held in in the case of wheel cylinders. This minimises the chance of air being trapped behind the pistons.
I had trouble bleeding 'S' calipers years ago and physically removed the caliper from the hub carrier so I could move the caliper around in my hand to ensure the bleed nipple was definitely at the highest point while bleeding.
I forget whether it was pistons pushed back or holding the caliper in my hand that solved the bleed problems for me.

[dklawson]

...have you fitted the calipers the right way round

In the opening post the OP states the caliper bleed nipples are at the top.

[Downton1960]

Once again thanks for the advice, I have been working on the mini today but mainly on wiring in the lights, fitting the whiskers and adjusting the suspension/tracking.

While I had the front wheels off I removed the pads one by one and allowed the pistons to extend out by just over 10mm, I lightly coated them in brake fluid and pushed them back in as advised.

I only had 10 minutes to bleed the brakes though as wife was heading to work, we starting at the rear furthest from the drivers position and worked back to the front by applying a very slow pedal depression. Each time I had clean bubble free flowing fluid but have had a minor improvement at the pedal where now the brakes once pumped to pressure some resistance remains after a few minutes which does actually prevent the car rolling, it is almost at the end of travel though.

I think if I did another cycle of bleeding it may improve some more but ran out of time.

I have ordered new a Gunson Eezibleed kit as hoping this will aid the situation and can’t wait to get my old one back.

Didn’t have time to make a servo bypass pipe today but will do this if I continue to have problems.

Thanks again, I will update again in the week if I get chance to have another go.

Dave

[dklawson]

You did not mention... is this DOT-3, DOT-4, or DOT-5 brake fluid. Which are you using?

[Downton1960]

I am using DOT-4 fluid

[EMOKID]

Bleed system and then try to isolate front calipers using pipe clamps or vice grips. Bleed 2 rear cylinders again and if pedal remains firm the caliper seals need sorted. If pedal goes spongy then bypass servo. If still goes spongy and not losing fluid at pipe joins or rear cylinders then master cylinder / seals faulty

[Downton1960]

Ok thank you, not sure the good ridge hoses will clamp though as very tough with braiding so think I would damage them?
I will make a servo bypass pipe up one evening this week as want to eliminate it being at fault.

What was the issue with the batch of faulty Cooper S master cylinders? Was it machining errors or just poor seals?

Dave

[roymck]

No don’t clamp Goodridge hoses .

[Maddog]

Did you bench bleed the master after rebuilding and before fitting?
Also, when I have had problems like this in the past, I have set up all four wheels to gravity bleed simultaneously (only needs 4 glass jars and four clear tubing lengths to suit) and leave the whole lot, even overnight. That often seems to work on the stubborn ones...