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MG Midget and Sprite Technical - Brake Master Cylinder & Pedal Travel

Just installed a newly kitted brake master cylinder and now something seems amiss
Mike Pelone

don't you have to centralise the shuttle otherwise it locks out half the system?
David Smith

We had a similar experience with a duel line cylinder which had been sent out to be rekitted (not midget, but Metro). Turned out that there was a difference in dimension which prevented the pushrod from returning properly by about 1mm. Result was that although the far end of the m'cyl (from the pushrod) would operate properly (as there is not initially a solid link between the two halves), the failure to return fully didn't uncover the hole linking the other half to the reservoir. Hence fluid wasn't being drawn through properly.

Symptom that gave the clue was a small inwards hiss when the bleed screw was opened, indicating a partial vacuum in the line ... set us wondering why and led to the cause.

May not be the case here, but with very similar symptoms it's worth checking.
Paul Walbran

Paul:

I will have a go at the rear wheel cylinder using a plastic hose to verify air movement on the return pedal stroke later today when my "lovely assistant" returns home...

When I was bench bleeding the master cylinder prior to connecting the steel brake lines - there seemed to be fluid moving into (and out of...) the reservoir when the pedal was moving back & forth. Could be that fluid is NOT flowing from the appropriate "back half" of the reservoir. Unfortunately, it might have to go back to the rebuilder since I have a lifetime warranty on the labor, etc.

Can you confirm that bleeding the rear brakes should give you the same brake pedal movement (ie., to the floor...) similar to bleeding the front brakes?

Otherwise, it just seems to me that the brake pedal "feels" like there is an internal obstruction to full pedal movement - again, another indication of an internal parts assembly problem...

Mike P.
Buffalo, NY
Mike Pelone

What david said, When bleeding brakes you must do light strokes do not put pedal to floor as you will move shuttle which will shut of fluid.
pd cumbers

pd cumbers:
well, I certainly did push the pedal to the floor when doing the front calipers...but that is what I have usually done for the last decade...

What is the best way to either relieve brake line pressure or re-center the shuttle valve within the pressure warning switch? Does it require any special tools?

The Haynes manual indicates that equalizing the pressure between front and rear brake lines should recenter the the shuttle valve. Does this seem right? If it is now off center - will it need mechanical assistance to get it back where it belongs? The dashboard light has never worked - I think that using a continuity meter directly on the pressure switch might be another way to detect the "central" position of the shuttle valve...

Thanks for any insight into this issue...

Mike P.
Buffalo, NY
Mike Pelone

very few if any MIdgets this side of the pond had the shuttle valve so you won't get swamped with answers from over here but I'm sure it's come up before; I expect an archive search will find the details you need.
David Smith

Hi Mike, i did the same as you at first pushing pedal to floor and could not get a good pedal.Then after some reading and thinking i tried again.I think the shuttle valve was returned to center after i undone the bleed screw on the valve houseing (could be wrong may have just gone back)This is what i done ,do up bleed screw on shuttle valve bleed back left side then front, then done the same on otherside so you end up with the one by master cylinder last. only using light strokes and (not) going to the floor. Well it worked for me.

Hope this helps. Paul
pd cumbers

O.K. - more fuel for the fire...

After posting an unrelated thread on the pressure differential switch earlier tonight...I decided to try one last test before packing up the rebuilt master cylinder and sending it back...

I disconnected each supply line to the brass pressure differential unit to compare pedal travel, pedal "feel" and volume of fluid displaced.

Very strange...when disconnecting the front brake supply line - the pedal has full travel and pushes out a large volume of brake fluid. In contrast, when disconnecting only the rear brake supply line, the pedal feels very firm [almost rock hard] and minimal fluid is pushed out. This was the same result I produced when I connected two short 6-inch brake lines to bench bleed the master cylinder when it first arrived...

My conclusion? The rear seals [cups] may be reversed on the piston that operates the rear brakes. This would explain the small volume of fluid displaced...I am still at a loss why the pedal feels as stiff as it does when the supply line is disconnected...

Anyway, I can now rule out an off-center internal shuttle valve blocking fluid flow. Tomorrow, it's time to remove the Master Cylinder to send it back. Summer is rapidly going away in Upstate NY and I have yet to put any mileage on the Midget...also, the wife is making noises about getting a used Miata...No support from the Home Team!

Thanks for any thoughts & comments..

Mike P.
Buffalo, NY


Mike Pelone

Heres some bad advice,,,Not sure how dangerous this is. so apply accordingly

being that you can get the front brakes to work good, and the front to rear bieas is 85 to 15...Im running 90 to 10 bias, and as you say summer is running out and the wifey is going all asia rebel on you

perhaps clamp off the rear brakes and put a T in the brake line to power the the front brakes...just becarful to not lock up the brakes in a hard braking skid so the back end comes around on you...aka no rear brakes

that way you can drive till the weather goes bad and when the snow falls then build the braakes correctly

just a thought...Like I said I WAS only relying on the rear brakes for 10% of my total brakeing capacity in a hard skid brake back when the car was running

just a thought

Prop...The jimmy rigger
Prop

Again, look up Rebecca's posts.
These MC have an odd lost motion connecting link between pistons, and they commonly break or are assembled wrongly. From your description, the MC is NFG. In any event, you should never get full pedal travel if either circuit is working correctly, since the entire point is that you will always have one circuit working, which means there must be pedal travel available for that to be possible. With both circuits bled and adjusted properly, opening a bleeder on either one should result in about 1/2 of the available travel being used, and the other half available for the still "good" circuit. It is not at all rare to find cars that people have been driving for years with only the front brakes working correctly. IF the rears are bled reasonably well and the link fails, the rears will work, but once you open the rear circuit and let the fluid out, it won't pull fluid back in so you get a half pedal front only situation - and they still drive it!

FRM
FR Millmore

Prop:
I don't disagree that the front brakes do most of the work in stopping a car...but, I don't want to be a published author who work appears in the local obituaries...

I have decided to remove the master cylinder and sent it to the Pros From Dover in Virginia. What good is a "lifetime" warranty if you never use it?

ps: drive fast but stop faster!

Mike P.
Buffalo, NY


Mike Pelone

This thread was discussed between 09/08/2010 and 27/08/2010

MG Midget and Sprite Technical index

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