To my other TR6 pages

November 18, 2015

Brake Servo

All TR6 cars had power assist brakes.  The power comes from engine vacuum collected from the intake manifold and stored in a reservoir in the brake servo unit.  The servo unit fits between the brake pedal and the brake master cylinder so that it can both sense pedal position and also augment the force applied to the master cylinder.

The servo unit on a TR6 is another one of those devices usually considered not to be a DIY item.  While these kinds of warnings amuse me, the servo is a little different.  Even if a determined person manages to get inside the unit, none of the internal parts appear to be available from retail suppliers.  There are commercial outfits who will rebuild the servos, so they apparently have the parts custom made for them.  Before laying a hand on my servo, I contacted one of these rebuilders about the possibility of buying internal parts, particularly the main diaphram.  They declined to sell me anything, citing "liability concerns", which doesn't ring true to me.  It seems to me that rebuilding or otherwise supplying entire brake servos would imply as much or more liability risk.

At any rate, I decided to forge ahead.  If I got to the point where the servo would be junk without some new innards, I could still just bag everything up and send it off to a rebuilder.  Even if it came to that, I could still powder coat the cases myself and save a little coin.

There is a wonderful resource on the Web for anyone considering opening a servo.  The technical pages at are a goldmine of information for many aspects of the TR6, including the servo.  I'm indebted to the folks at Buckeye for providing this valuable asset.  

Commensurate with the rest of the car, my servo was a greasy, dirty, rusty lump.  Notice the debris inside the nose around the fork.  That is the remains of a filter.  More on that later.

There are a few things that can be easily taken off the unit--the spacer, the dust cover, and with a little more dermination, the front seal and the "non return valve" where the hose from the  manifold connects.  But for the really interesting stuff the case has to be opened.  

The case is in two halves, joined by a bayonet style joint.  The circumference of the internal rubber diaphragm is captured in the joint and is under considerable compression, so the joint isn't a pushover.  I made some tools similar to those described on the Buckeye site.

With a cheater bar and some enthusiastic hammer work, the joint finally gave up.  Imagine my happy surprise when I removed the internal diaphragm/carrier assembly and saw that the inside of the cases were in pristine condition!  Other than some congealed 40 year old grease, the inside surfaces looked brand new.

Encouraged, I turned to the internal parts.  The large rubber diaphragm is carried by a large plastic (phenolic, I think) carrier.  Inside the central bore of the cerrier is a valve assembly.  The valve assembly is located and retained in the bore with the little forked key.

The diaphragm appeared to be intact, and was still very supple, but had an uneven surface coating.  I'm guessing it was the parting compound from the molding process.  After a little cleanup, it looked much better.  I couldn't see anything wrong with the diaphragm.

The valve assembly appeared to be staked together so I didn't disassemble it.  I believe when these were available, they were sold as a unit.  

There are two rubber parts in the assembly.  One acts as a seal inside the bore of the diaphragm cerrier, and the other seats on an opening in the carrier that connects the chambers on either side of the diaphragm.  With the brake pedal not pushed, the volumes in back of and in front of the diaphragm are connected, so that the engine vacuum evacuates both chambers.  When the pedal is pushed, first the valve assembly closes off the connection between the two chambers, and then opens the rear chamber to atmospheric pressure through the cetral bore in the carrier.  The imbalance in pressure pushes the carrier forward, adding force to the pedal force.

The bore in the carrier was shiny and smooth, and the valve assembly seemed to still be a nice fit.  A little rubber preservative made the diaphragm look even better.   I was pretty encouraged at this point.

I cleaned up the outside of the cases and powder coated them with a semigloss black.  

This assembly seals around the neck of the diaphragm carrier.  Again, the cleaned and lubricated rubber seemed to still be a good fit.

The rest of the parts, all original.

This dust cover is one of the parts that is available for the servo, but after I cleaned it up, I could see no reason to replace it.  It was still pliable, and a good fit on the case.  If I got this cover in the mail as a replacement, I would be totally satisfied with it.

The cases went back together much easier than they came apart.  Some rubber lube on the diaphragm circumference helped a lot.  The white pads protect the paint.  I tested the check valve before I put it back in.

The last item was the filter inside the nose of the carrier.  It filters the air drawn in to the rear chamber when the vacuum is released.  It was the only really perished part of the entire servo.  It is not available anywhere that I could find.  In the end, I bought a piece of 1" thick polyurethane open cell filter media, and cut a cylindrical piece out of it.  It doesn't look too far off from what's left of the original in the middle picture.

So here is the critical question:  Can all of these 40+ year old rubber parts still contain a vacuum?  I pumped the unit down to 15 inches of vacuum, and turned the pump off.  It looked pretty steady.  One hour later, the vacuum was a little under 14 inches.  In the interest of full disclosure, I will point out that since I didn't have another vacuum gauge, I left the valve from the pump open so I could use the pump's gauge.  This added the pump's reservoir volume to that of the servo, so it's possible that the servo alone might leak down a little faster.  I was hesitant to actually actuate the servo because I wasn't sure how it would react without the load of a master sylinder attached.

I really couldn't have hoped that this exercise would have turned out better.  It is still hard for me to believe that these 40 year old rubber parts are still doing the job so well. Total cost for the servo was close to nothing.  Some power coat powder, some silicone brake grease, and $8 for the filter material.

Comments to Ed at:

To my other TR6 pages