14 replies to this topic

[Moose94]

Afternoon All,

 

Setup:

1990 Mini

Brake Servo - Bulkhead mounted with twin output master cylinder to brake bias valve (standard).

Servo rebuilt with new diaphram and seals, vacuum line attached to Inlet Manifold.

Entire brake setup new from Servo onwards, New lines, calipers, drums, flexi connections, Briaded lines from Master to Bias. I did rebuild the servo to try and fix this problem but it has not worked (new diaphram seals etc).

 

The top inch or so of my brake pedal is very airy and doesnt provide any braking power, past this point the brake pedal is firm and the car stops as normal. This is consistent when the car is running, however the pedal is firm when the engine is off.

 

Ive bled and bled and bled the system and there no air in it.

 

Ive been around the servo and used brake cleaner to hunt for any sort of air leaks in the system and the car has not changed idle at all.

 

Sometimes when pulled up at traffic lights and on the brakes, the idle will hunt slightly indicating there is a leak or something changing the pressures in the inlet somewhere, when on just the handbrake the engine idles fine.

 

This afternoon ive disconnected the vacuum line from the inlet and taped up the banjo bolt on the manifold and gone for a drive. Pedal is heavy with no movement before brake force is applied, confirming that the system is bled correctly.

 

Am i missing something here or is there an issue with the Servo itself, its almost like when the engine is on its pulling a large amount of vacuum onto the servo, so the initial press of the pedal is just picking up the slack of springs in the system until the pedal plunger in the servo reaches the servo diaphram.

 

The engine is very tuned, Bored/Cammed/Ported and at around 90hp, would this be causing excess vacuum on the servo pre engaging the brakes?

 

Worst comes I will  purchase an older pedal, double spring and a vertical master cylinder and ditch the Servo, obviously letting my insurance know, bonus right leg workout.

 

Any help would be great, thanks!

 

 

[alpder]

The initial press of the pedal takes up the slack in the brake shoes and pads. Once the slack is taken up, only then you start to feel firmness of the pedal. So correctly adjusting the rears is essential to getting the best pedal feel.

 

And also checking there's no run-out of the disks (which would push the pads apart - but that's unlikely to be present because you'd already know about it due to juddering and/or locking-up).

 

If you're bleeding solo, my experience is that the Gunson Eezi-Bleed just doesn't have the oomph to always drag out all the air. Having someone also stab the pedal down helps get the last bubbles out of things like the pressure regulator which (on a single-circuit car at least) has outlets lower than the highest point of its chambers.

 

Also, if the air vent in the reservoir is blocked, I suppose in theory that could cause the fluid to be sucked back and the pistons to be retracted by the partial vac created in the reservoir. A thing that I've only ever experienced on a push-bike, which has a tiny reservoir and gets exposed to a lot of dirt. Exciting, though.

[sonscar]

Cammed and ported in my experience tends to reduce vacuum at idle.Pump the pedal until hard and holding it down start the engine.the pedal should go down if the servo is ok.Holding the brakes on can affect the idle as the servo works.What you describe sounds normal to me but I am not there to see.Steve..

[sledgehammer]

I would clamp the flexi on each corner , one at a time ,

 

to see if you can isolate a corner , if there is a difference it's either air or adjustment .

Is it possible that the bias valve is playing up ?

 

I would see if you can bleed the unions coming out of it , as it is at a high spot in the system .

 

also are the rear brakes working on the pedal ?

 

as above - higher performance cam , often gives lower inlet manifold vac ,

 

but not sure the servo is your problem .

 

best of luck

Edited by sledgehammer, 14 March 2025 - 09:04 PM.

[Moose94]

The initial press of the pedal takes up the slack in the brake shoes and pads. Once the slack is taken up, only then you start to feel firmness of the pedal. So correctly adjusting the rears is essential to getting the best pedal feel.

 

And also checking there's no run-out of the disks (which would push the pads apart - but that's unlikely to be present because you'd already know about it due to juddering and/or locking-up).

 

If you're bleeding solo, my experience is that the Gunson Eezi-Bleed just doesn't have the oomph to always drag out all the air. Having someone also stab the pedal down helps get the last bubbles out of things like the pressure regulator which (on a single-circuit car at least) has outlets lower than the highest point of its chambers.

 

Also, if the air vent in the reservoir is blocked, I suppose in theory that could cause the fluid to be sucked back and the pistons to be retracted by the partial vac created in the reservoir. A thing that I've only ever experienced on a push-bike, which has a tiny reservoir and gets exposed to a lot of dirt. Exciting, though.

 

Ill have a go at adjusting the rears again, they were ok a few months back when the whole lot was rebuilt but something may have moved in the last hundred miles or so. Ive got one of those pressurised pots to bleed the system that locks onto the top of the master cylinder. There doesnt seem to be any wheel wobble from the front or rear under normal driving or braking, and certainly nothing odd when ive disconnected the servo.

[Moose94]

Cammed and ported in my experience tends to reduce vacuum at idle.Pump the pedal until hard and holding it down start the engine.the pedal should go down if the servo is ok.Holding the brakes on can affect the idle as the servo works.What you describe sounds normal to me but I am not there to see.Steve..

 

Yeh following the logic of where the air comes from in the system i can understand a certain amount of idle hunting with a servo, it used to do it before the engine was built, now its way way worse. Ill have a go at starting the car with the brake down and see if that does anything odd.

[Moose94]

I would clamp the flexi on each corner , one at a time ,

 

to see if you can isolate a corner , if there is a difference it's either air or adjustment .

Is it possible that the bias valve is playing up ?

 

I would see if you can bleed the unions coming out of it , as it is at a high spot in the system .

 

also are the rear brakes working on the pedal ?

 

as above - higher performance cam , often gives lower inlet manifold vac ,

 

but not sure the servo is your problem .

 

best of luck

 

The rears are good, gave the car a good stamp on the pedal last night and the rears locked up nicely, and it braked straight. 

 

If the inlet has a more neg pressure that would pull the diaphram towards the master cylinder more right? So my initial pedal input would be joining up the slack in the rods and diaphram on the servo bracket side of the system? Ive had a play with adjusting the connector rod on the bulkhead side of the servo to get the brake pedal in a nice ish position in the interior, but ive run out of travel to an extent, would a longer adjustment shaft help this in effect matching up the diaphram mechanically to where its been vacuum'd into the master cylinder when the engines running?

[sonscar]

Be careful adjusting the pedal rod.If there is not enough play then it is possible for the piston to not retract far enough to release pressure and the brakes stay on.Steve..

Edited by sonscar, 15 March 2025 - 02:16 PM.

[NLinPEN]

Just a suggestion: my Mini has an inline non-return valve in the vacuum hose that connects the manifold to the servo. This valve was not working well and needed replacing (I installed 17H2646). Have you confirmed that, if your car has such a valve, it is working correctly?

Edited by NLinPEN, 15 March 2025 - 11:28 AM.

[alpder]

Not an expert on Mini brakes, but rears locking-up is A Bad Thing on most cars, generally offering the driver a backwards introduction to a nearby hedge when braking on any significant curve. Essentially an involuntary 'handbrake turn'. Most (all?) Minis have a rear brake regulator to prevent this. It may be prudent to check the regulator is present and is working. I suppose it'd be possible to dispense with a regulator, but only if using very small bore rear slaves compared to the fronts.

 

One of the described problems is excessive brake pedal travel. Pedal travel is minimised when all the drum brakes on the car are perfectly adjusted. (Disks are self-adjusting.) Unfortunately, testing that they work doesn't necessarily also test that they're adjusted - they'll work fine even if they're adjusted a bit slack, but the pedal will have to move further to bring them to the bite point.

[Moose94]

Just a suggestion: my Mini has an inline non-return valve in the vacuum hose that connects the manifold to the servo. This valve was not working well and needed replacing (I installed 17H2646). Have you confirmed that, if your car has such a valve, it is working correctly?

It does have this yes, ill have a play around with it, im guess it can be tested simply by sucking/blowing on it to see if there any resistance.im guessing the return should stop air leaving the servo system rather than entering.

[Moose94]

Not an expert on Mini brakes, but rears locking-up is A Bad Thing on most cars, generally offering the driver a backwards introduction to a nearby hedge when braking on any significant curve. Essentially an involuntary 'handbrake turn'. Most (all?) Minis have a rear brake regulator to prevent this. It may be prudent to check the regulator is present and is working. I suppose it'd be possible to dispense with a regulator, but only if using very small bore rear slaves compared to the fronts.

 

One of the described problems is excessive brake pedal travel. Pedal travel is minimised when all the drum brakes on the car are perfectly adjusted. (Disks are self-adjusting.) Unfortunately, testing that they work doesn't necessarily also test that they're adjusted - they'll work fine even if they're adjusted a bit slack, but the pedal will have to move further to bring them to the bite point.

The rears locking is only really present when its really really pressed, ill have a go at adjusting the rear drums again to make sure they are setup correctly and see if that resolves some of the brake pedal feel.

[NLinPEN]

 

It does have this yes, ill have a play around with it, im guess it can be tested simply by sucking/blowing on it to see if there any resistance.im guessing the return should stop air leaving the servo system rather than entering.

 

Yes, your guesses are correct.

[alpder]

Several threads on here about drum adjustment - one of them a sticky - if you need guidance. Key points are to adjust things (including the h/b cable) in the correct order and to expect drums to slightly drag when correctly set.

[sonscar]

Measure the diameter of the shoes when fitted and compare with the drum diameter.I have had shoes that are so badly sized that they would not adjust correctly,the fix for this was too agricultural to admit to on here.Steve..