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MG Midget and Sprite Technical - All 4 wheels locked first time out after rebuild

Hi all, looks like my Frog won't be going on it's first outing to a car show on Sunday unless anyone can solve this problem for me.

Converted from drums to discs so everything is new, and I mean everything, even the brake pipes are new.

Took it out for the first time today to get some petrol, only about a 2 mile round trip but by the time I got back the brakes had started to lock up. I thought it was only the back brakes as I had nipped up the adjusters quite tight but when I jacked up the front the front wheels were very reluctant to turn. I opened a union at the 5 way point and brake fluid squirted out. After that I could turn all 4 wheels easily.

I have silicone fluid BTW but I wouldn't have thought that would be the culprit.

Any help gratefully accepted.

Rob aka MG Moneypit

I've only general advice until someone better comes along

if it's all four wheels then it sounds like the hydraulic system

start with the master cylinder, if it was just the front it could involve the flexi-hoses, is your MC with the clutch too

only thing I can think of is if you need and haven't got a hole in the MC cap or it's blocked but I'm sure my MC hasn't a hole
Nigel Atkins

It's a combined one, the type used for Disc brakes and made by AP Capro (I think, AP certainly). Everything is new, cylinders, pipes, flexi-hoses, pads, discs, shoes, everything, but I do agree it sounds like the MC.

Rob aka MG Moneypit

Sounds like either a faulty M/C (piston valve wrongly assembled?) or the brake pushrod is wrongly adjusted. It needs enough adjustment so that the M/C piston moves back past the equalising hole in the cylinder, to allow fluid to flow back between each application of the brakes. Otherwise each time you brake you will pump more fluid into the pipes and the brakes won't release.
Guy Weller

sounds like a good possibility of incorrect adjustment - you will be going to the ball!
Nigel Atkins

If you have fitted new pushrods to the master cylinder then they may need trimming. At standstill there should be a little clearance. If using Mk1 backplates you will need MM slave cylinders.
If it isn't the pushrods then it may be worth posting on the MGA site as they have had problems with master cylinders but AP caparo are considered there as the best.
Alan Anstead

I tried the brakes again before I saw all the replies and I can feel the pedal jacking up with repeated presses and after doing that the front wheels were difficult to turn again.

I have used the pushrods from the drum brake MC and didn't check whether they needed adjustment so Guy you might be right. Which way do they need adjustment? Do they need shortening to allow the master valve to return more? I guess that might be the way it works.
It sounds very plausible. Pushrods too long me thinks. Any one confirm this before I get my hands dirty.

Rob aka MG Moneypit

Rob, sorry I cannot give any more (reliable) advice 'cos I haven't got that far in my Frog rebuild yet! Alan will know better, and be wiser. I would assume they rod needs to be shortened although Alan's message sort of implied they weren't adjustable and you may need to shorten by selection of parts, or maybe with a grinder?
Guy Weller

OK Guy, thanks.

I tried shortening the pushrod using the maximum adjustment possible but it made no difference. However, I took the clevis pin out and moved the pushrod out a bit more and the pressure went away. So I put the clevis back, pumped up the brakes, made sure the front wheels were locked, took out the clevis pin, released the pushrod again and I was able to turn the wheels once again.

So I think the problem is the length of the pushrod. So using the drum brake pushrod was not a cost saving idea!!!!

If Alan is still on-line do you know the lengths of the disc brake pushrod and the drum brake pushrod so I know how much to trim off?

Rob aka MG Moneypit

have a look in the Archives or recent threads as I'm *almost* sure this was covered recently
Nigel Atkins

I dont know the exact measurements but as the rod is adjustable you should be able to gauge the length by where it is at present and where it should be with the piston at rest, cut a suitable amount off, and then fine adjust with the thread and locknut. The rod wants a couple of thou clearance when the piston is at rest.
Alan Anstead

I had exactly the same problem when I converted my Frog to discs, brakes dragging and stinking hot after a couple of miles.
Problem was the valve in the brake mater cylinder that retains a small pressure in the system - for drum brakes. I removed the valve and problem solved.
I note that you are using an AP master cylinder but may be worth checking if it has the same valve. A very long nosed plier needed to remove it.
D Stanfield

Hi everyone, thanks for all the help. It looks like the Frog is going to the ball

I set the pushrod to minimum length but it was still the same. I read the manual (RTFM or what?) and read about the free play of 0.8mm min at the pushrod but even set at minimum there was none. (interestingly, in the Haynes manual all this is in the Clutch chapter, no wonder I couldn't find it before!!)

Anyway, I cut 4mm of the pushrod and put a radius on the end. Now I have got the required free play and more importantly, the brakes don't bind. The clutch pushrod has no free play but the clutch master works differently so doesn't stop it from working.

There is only one pushrod for all marks with tandem master cylinder so something must vary caused by the manufacturer. Maybe the mounting holes were drilled a few mm too far back or the pistons are a few mm longer than original or both? It just shows how careful we need to be in finding all this out and passing on the knowledge.

I generally pass this back to whom ever I get my bits from but I'm afraid to say there attitude usually stinks. Some day, someone is going to have cause to sue a supplier, maybe then they will listen and take our feedback seriously.

Thanks again for all your help.

Rob aka MG Moneypit

well done Rob

of course my vote goes with RTfM but even the best of us can slip up occassionally
Nigel Atkins

Well they say there ain't no such thing as coincidence

then we get cases like yours

quite often

I think Alan posted that when they produced the new make master cylinder they made newer shorter pushrods too, the recess inside the cylinder body is shallower than the originals

Funnily enough many years ago when I got my car on the road, first time out I drove to a team meeting about seven or eight miles away

guess what happened ;)

Yes I needed to impose a little adjustment to my rebuilt master cylinder. :)

I was brought to that familiar standstill and when I looked the discs were glowing orange and one of the track rod end gaiters was on fire

Exciting few moments ensued :)

Glad you got sorted, enjoy your show


for clarity

I meant to type

the recess inside the piston is shallower than the...


Do you recognize this part? and dit you use it? You need it if you transfer to disks with the original MC. See the comment of Guy and Dennis.

Flip Brühl

God I truely hate who ever came up with the concept of a spirl clip...they should be beaten, set on fire, then shot in the head for good measure

Of coarse haynes was no better... "remove spirl clip" to install sprial clip is the reverse of remove...gee thanks captian obvious, any other bright ideas

That was a 2 week adventure just to learn what a sprial clip even is, let alone how to remove it

Tilton... my lord and savior

Prop and the Blackhole Midget

Hi Rob

Some useful background info on pushrod lengths of combined master cylinders for drum braked Mk1/2 Sprites and Mk2a disc braked Sprites, and conversion drum to disc:

M Wood

Hi guys, all ok now with shortened pushrod. Went to a show in it and it performed well. Will post a picture when i have a chance to download my camera.

Flip, I do recognize that part but I assume that as the master cylinder was new AND intended for disc breaks it SHOULD have all the corect bits. What I didn't allow for was the "modernisation" of the master cylinder requiring the use of a shorter pushrod and the fact that the supplier didn't tell me, either because he didn't bother (or didn't know) what he was selling.

But thanks again everyone. Got me out of a tight situation.

Rob aka MG Moneypit

This thread was discussed between 24/05/2014 and 25/05/2014

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