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MG Midget and Sprite Technical - Pesky brake non-return valve

I converted my Frog to front Disks years ago and never removed the Master cyl non-return valve, I've never noticed problems with the brakes binding but on a spirited drive the other night to check all was well for the weekend the brakes started binding, by the following morning they were fee as usual.
I don't want the risk of this giving me problems on the journey to Gaydon so today I'm going to pull the M/C and remove the valve, can anyone point me to a diagram or picture of what I'm removing, just to be on the safe side? I'd hate to pull it apart only to find that I don't know what the valve is and end up shoving it all back together as it was in the first place!!

Thanks
Mark
Mark

I didn't think there was one so this ought to be an interesting and educational thread.
Bob England

Bob, I spent ages (apologies to my boss) searching the archive and non-return is obviously an incorrect term, I should be saying 'residual pressure valve' instead I guess?
Anyway, I'm sure this is my problem, under regular christian driving its not a problem, but under repeated heavy braking sessions, the residual pressure is building up in the calipers too much hence binding brakes.
From the threads I found (and there were loads!) it should be really obvious that I have some form of plastic valve at the end of the brake cylinder, lets hope it is this, otherwise I'm going to be a little concerned on the journey tomorrow.

Mark
Mark

To keep your demise topical and to throw a spanner into the logic

If it was the "'residual pressure valve" then why would it not hold pressure after braking normally. I believe that brakes comming on when used in heavy braking is due to heat build up in the fluid which normally escapes through the vent in the master cylinder as does the pressure in the system when the pedal is released.

Therefore I would expect that if the RPV were faulty it ought to hold the brakes on whether or not they were being used hard wouldn't it?
Bob England

It could be crud in the vent hole but somehow I doubt it. Two things you might want to check before you pull out the Master Cylinder is that the pushrod at the M/C has got the correct free travel before it hits the piston - otherwise you will pressurise the system, and that the flexible brake hoses ( front and rear) arn't collapsed internally producing their own one way valve of sorts.
f pollock

The all-drum set up had a residual pressure valve, approx 8 psi, to make sure that the seals and pistons were kept in place.

The later twin piston rear cylinder that came at the same time as the front discs has a spring that does a similar job.

If the valve is not removed, the low residual pressure can be enough to keep the pads rubbing on the disc, and produce a lot of heat (just ask our local tyre fitter before I realised I had not removed it!!). I went to have the front wheels balanced and the hot wheel nuts left a 24 tpi impression on his thumb and first finger! Not very amused.

Mark,

I don't think it could cause a noticeable 'bind' by itself, have you checked the pedal to master cylinder free play adjustment?

Richard
Richard Wale

>>> ...and that the flexible brake hoses (front and rear) aren't collapsed internally producing their own one way valve of sorts. <<<

While I've never had this happen myself, the situation has come up any number of times during the 11+ years I've been hanging out on this BBS, and collapsed hoses were repeatedly proposed as the culprit.

And like Bob, I'm not familiar with any kind of non-return valve in the MC.

Best of luck,

-:G:-
Gryf Ketcherside

Mark, a couple of things to check. First check the pedal for free travel. I've had the pivot tube start binding and not allow the pedal to return fully. For a later car with the mechanical brake light switch it will leave the brake lights on and may also keep the piston in the master cylinder from returning far enought uncover the fluid return port in the master cylinder. A poorly adjusted mechanical switch will cause the same problem with the piston, but I figure you're still using the original pressure switch in your Frog.
I believe that the residual pressure valve is built into the inner pieces of the cylinder. It's the item circled in red in the drawing I believe.


Bill Young

This little thing?

I installed it on my drum to disc conversion, and no problems yet.


Nick

Thanks for all the replies, after an afternoon working on the car, I pulled the M/C and stripped it down, it has no valve of any sort, so I've either removed it in the past or it never existed.
some very dark fluid came out of the M/C and I *think* that one of the very small holes may have been blocked, I cleaned them both out anyway.
All reassembled and also wound out the pushrods and adjusted them correctly, problem has gone away :o)
The brake pedal travel is further than I would like ( I guess this is the purpose of the valve) but it has a nice progressive feel to it, it was all bled through with lots of new fluid and an easybleed, so deffo no air in the system.
In the past the pedal was just rock solid and not confidence inspiring. So, fingers crossed all is now well and roll on tomorrow!!

Mark
Mark

This thread was discussed on 23/05/2008

MG Midget and Sprite Technical index

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