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MG MGB Technical - brake master cylinder with pwda


poor brakes----
getting ready to replace and reassemble the innards of the tandum master cylinder with pdwa.

is it necessary to replace the parts of the pressure differential unit, item 16 in the haynes mgb manual, page 157 ??

i am prepared to do so, but it looks to me from the diagram that there should be very little movement of it and hence little wear, or am i overlooking something ?

john sutter

No, I think you sum up correctly. I did not replace this unit - it doesn't do anything for the working efficiency of the brakes. You may find that replacement of all the hoses (which become soft with time), has more effect.

good luck

R Walker

John. The pressure differential segment of the master cylinder has two rubber O rings in it. (This latter from memory--there may be more, but I only remember two.) These O rings deteriorate over time and, when that happens, allow air to be drawn into the master cylinder's main reservoir(s) and allow brake fluid to leak out through the switch on the bottom.

I know of one instance where a 77B, using the same master cylinder, was taken to a local MG shoppe, four times, because of bad brakes. In each case, they were charged for a "master cylinder rebuild". And, in each case, the differential pressure section was not rebuilt. When I examined the car, the first thing I noticed was the leakage from the pressure differential switch. Removing the switch and plugging the hole with a short bolt corrected the problem until the parts could be secured to do a proper rebuild.

Yes, rebuild the pressure differential portion of the master cylinder or you may experience problems later--assuming that you are not having them now and did not notice.

Les Bengtson

On this subject of brakes, a tecnical question. A previous thread I read was a gentleman who was researching the possibility of twin caliper pots from an Austin Princess to the MG stub axle. Now this mod is not new, but if one raises the efficiency thus of the front end does one have to alter the 'ratio' between front and rear? And how is this done? Mike
J.M. Doust

I have another thread about a similar job, rebuilding my 78B tandem master cylinder with PDWA. Amongst other questions I've had, how do you remove it?
I'm following the Hayne's manual and it tells me to extract it using air pressure through the primary outlet port. I've tried air pressure through every port (I considered a few unpleasant ones for the authors of the book) and no luck.
Everything else came apart easily. I've used an entire can of brake cleaner and also soaked it in WD40 overnight. It aint budging.

Lucky for me it doesn't seem to be leaking at all, but I still don't like the idea of it being stuck inside when I'm going to the trouble of renewing the rest of the innards.
D O'Brien

If the equalizer assembly does not come out of the chamber easily it is for one of two reasons:

First, it has rusted in place.

Second, the rubber O rings have hardened to the point they are binding.

Let us look at this subassembly of the master cylinder. In the front is a 13/16" short bolt (14) with a copper washer to seal the system. When the bolt is removed, there is a slotted spacer (15) of 3/4 round design with two slots cut into the spacer opposite the open segment. This can be removed with a hook e.g. a dental pick. Behind that is a single, steel rod upon which are placed an E clip, a spring, another E clip and two rubber O rings (16). This is from Fig. M.1 in section M of the factory workshop manual which shows these parts above the illustration of the main channel parts. This should allow anyone to see what parts are there, in most manuals, so we understand what needs to be done.

The rod has a groove cut into the forward end which allows the E clip to hold the spring from moving forwards. A thin pick e.g. a modified dental pick or a piece of steel welding rod, et. al., can be formed to provide a short, hooked end which can then be inserted into the front of the opening, hooked into the groove in the rod, and used to move the rod forwards and outwards.

The only times that I have seen this method fail have been when the system was allowed to go too long without changing the brake fluid (which attracts water), allowing the rod to rust inside the bore. In such cases, a good quality penetrating oil may (or may not) allow the rod to be loosened.

One note. WD-40 is not a penetrating oil in the sense that is needed here. It does penetrate, but will tend to dry, leaving a varnish that gums up the moving parts and can exacerbate the problems with stuck parts. ZEP Restore is the best rusty parts penetrating oil I have found while others have found, I believe, a product called PB Blast to be effective. WD-40 is a very limited use item, best left to its original purpose of displacing moisture. It is not designed to be a solvent for rusted, or otherwise stuck, parts. The varnish left behind by the drying of the WD-40 forms a barrier to further moisture contamination (which would cause rust) and is intended to be removed, by chemical cleaning, before the protected parts are machined or used.

Les Bengtson

Brake mods: Remember it is the tyre contact patch that should be the overriding limitation on retardation rate. As long as you can lock the fronts with gradually applied pedal pressure, the biggest and best discs and pads in the world will make no further difference - *except* for competition use in preventing fade. If you put massively wide and sticky tyres on then you probably will need to uprate the callipers so you can lock the fronts again.

Also remember that you more you reduce pedal effort to lock the fronts by changing the callipers, the worse your overall braking gets unless you *do* alter the front/rear balance, as you are also reducing rear braking effort. The easiest way of increasing rear braking effort - for a roadster anyway - is to fit GT rear cylinders (although note that V8s had roadster rear cylinders), as they have a larger diameter than the roadster items. They have a differently positioned locating pin, so the backplate would need to be replaced or drilled to accept it.
PaulH Solihull

This thread was discussed between 08/10/2011 and 10/10/2011

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