View single post by Mark Rosenbaum
 Posted: 03-17-2005 09:36 pm
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532

Originally posted July 2003.


Recently the master cylinder (M/C) in my JH began exhibiting unmistakeable signs of incipient failure. Having heard that the success rate when rebuilding these is quite low, I decided that a new M/C was in order. It's fairly well known that the TR6 M/C is a drop-in replacement for the original JH part, and according to the pricing information I have the TR6 part is several hundred dollars cheaper.

Since Delta Motorsports doesn't offer TR6 M/Cs, as far as I know, I checked the archives of various British car bulletin boards to locate a reliable source. Concluding that The Roadster Factory had the best overall reputation, I ordered the part from them. Their catalog price was $200, with shipping another $10 or $15, but as it turned out they were having some sort of sale, so the part was $180 and shipping only $5. Unfortunately the part was out of stock and had to be ordered from England. This took a couple of weeks.

Since I'd been treating the studs securing the M/C to the brake servo with Liquid Wrench on a fairly regular basis for several years, removing the M/C was not a problem. Knowing that a great deal of brake fluid would surely be spilled in the engine bay, I lined the left side chassis rail with a thick wad of paper towels backed up by shop rags, and had a large metal coffee can immediately at hand to accept the old M/C once removed.

I used a flare wrench to loosen the ends of the two pipes connecting the M/C to the PDWA. Once these fittings were merely finger-tight, I removed the two 3/8"-24 tpi nuts that hold the M/C to the brake servo, then finished removing the brake pipes. The two brake pipes were then removed and followed the old M/C into the coffee can. (These pipes were removed solely to allow easier bench bleeding of the M/C, as described below.)

There was considerable surface rust on the forward surface of the brake servo where the M/C had sat, but this was removed fairly easily with a bit of wire brushing and the aid of an aerosol can of Berryman's brake cleaner. The M/C's bore, however, showed signs of somewhat heavier rusting, and I surmise that some of this rust was carried into the cylinder when I last bled the brakes, eventually causing the M/C's internal seals to leak.

This is a messy process -- REALLY messy if you spill any brake fluid -- but is highly desirable as it cuts down dramatically on the bleeding needed at the wheels. According to some sources, bench bleeding is also necessary to prevent damage to the M/C while bleeding the system after installation.

While one can go to considerable extremes to bench bleed a M/C, what I did was to connect the two M/C-to-PDWA pipes to the M/C, then connect a foot-long piece of hose to the free end of each pipe. I then propped up the M/C in one of the drawers of my tool box, and inserted the free ends of the two hoses into the M/C's fluid reservoirs. Then I filled the reservoirs with brake fluid and used finger pressure to gently depress the M/C's piston until only fluid was emitted from each of the hoses. The process took about five minutes. (Cleaning up the mess that occurred when one of the hoses came loose and piddled brake fluid all over my tool box took considerably longer.)

Though I normally prefer DOT4 brake fluid due to its much higher wet boiling temperature, the cap of the TR6 M/C says to use only DOT3, so, after a bit of pondering, that's what I ended up using. Hopefully this won't cause the new M/C to self-destruct.

Carrying the somewhat soggy M/C from the workbench to the car, I slipped it into place on the brake servo studs and installed the mounting nuts and washers not quite finger tight. The two rubber hoses used in bench bleeding the M/C were removed, one at a time, and the end of the associated fitting screwed into the PDWA. Once again, this is a messy process as fluid will be leaking from the M/C out the end of the pipe until the fitting is fully seated. The stud nuts were then torqued down (I couldn't find a spec for this, so picked 20 ft-lb), and the brake pipe fittings were tightened with a flare wrench. A quick press on the brake pedal resulted in a fairly firm pedal with the expected slightly spongy feeling one expects when there's air in the system. Bleeding the brakes took care of this.

I used copious amounts of aerosol brake cleaner to flush away all of the brake fluid that leaked out during the swap, and slid the rubber tube that fits over the clutch cable to a location where it would keep the cable from abrading the brake fluid reservoir.

In conclusion, this really is a simple swap without any hidden problems as long as the nuts securing the M/C to the brake booster aren't rusted in place.

Finally, thanks again to everyone who responded to my request for tips concerning the swap.

Mark Rosenbaum Kingman AZ 74 JH 16371


Dull Boring Technical Stuff


TR6 M/C.

The Roadster Factory's description for the M/C was 'T56 Master Cylinder Assy, Brake', with part number GI 64068822. The part, when received, came in a green Lucas box which also bore part-number-like markings of 'PMF 214' and 'GMC 234'. The casting number on the metal part of the new M/C was 64678258. A red plastic strip secured around the cylinder read '68822' which is obviously the last 5 digits of the Girling part number. This was followed by '6040' which, I am told, relates to the fluid percentages delivered to the front and rear brakes respectively. Next came the letter 'C' and finally the code '150P'. I don't know what either of these means.

Original JH M/C.

The factory item is Girling part number 64066027. The part on my car is either original from the factory or a long-ago replacement. It has a red-orange plastic strip bearing first the number '6027' then '08' then '60/40' then '97GS'. The plastic fluid reservoir bears casting number 77470129 while the metal body bears casting number 64676942 followed by '027' and a very large 'C' (which might perhaps actually be a G, as in Girling).

Brake Pipes.

There are two 3/16" diameter pipes that connect the M/C to the PDWA. Each pipe has a bubble flare and a male fitting on either end. These fittings have different sized threads as noted above, but in all cases their heads are 7/16" across flats. The shop manual claims these are 10mm across flats, but this was not the case on my car. For the aft port, the line was about 18" long and had a 7/16"-20 tpi male fitting on either end. For the fore port, the line was about 13" long and had a 3/8"-24 tpi male fitting on either end. The stock pipes fit the TR6 M/C without any problems whatsoever.

M/C Dimensions.

I measured various locations on the TR6 M/C as indicated below. In all cases, these dimensions match those of the original M/C.

Flange bolt mounting centers 3.22" to 3.25"

Flange bolt hole diameters 0.42"

Flangeside cyl. protusion OD 1.60"

Flangeside cyl. protrusion ID 1.00"

Fore Port (rear brake) threads 3/8"-24 tpi

Aft Port (front brake) threads 7/16"-20 tpi

I would think that any M/C that could be made to comply with the dimensions above should bolt up to the stock brake booster and work well enough. The two port threads are, IIRC, standard on the M/Cs used in most American cars, so there ought to be a horde of potential replacements if one is willing to fabricate a couple of brake lines. I will also note that European JHs don't use the PDWA at all but instead use a coupling for the rear brakes, and a tee for the front brakes.