K14 – Servo Survival

by Phil Lindsay

The vacuum-power brake servo unit is really a two-sided beast. Its good side provides substantial braking assistance to romping Tigers and, thus, it seems to be desirable for most Tiger owners. I’ve only heard of a couple Godzillas who don’t need the servo. The bad side of the servo can really be nasty, like engine vacuum leaks (air to vacuum chamber leaks), loss of hydraulic pressure (internal fluid leakage in piston seals) or loss of hydraulic fluid (fluid goes into engine intake due to vacuum seal leakage). Rebuilding the servo requires more effort than the brake cylinders, but the extra effort is offset by the cost of a new servo ($150).

The rebuilding effort should go well if your brake system has been cared for during the servo’s life. I think that the most important factor is periodic changing of the brake fluid. I don’t mean topping off the reservoir every year. You’ve got to completely purge out the old fluid.

Start by emptying the master cylinder reservoir with a squeeze bulb and wipe out all of the dirt with a clean cloth. After refilling with fresh fluid (half-opened cans of unused fluid are not considered fresh if there has been moisture contamination), start bleeding each brake in the usual manner.

Continue to pump at each brake until clean fluid comes out; don’t forget to replenish the master cylinder reservoir. This entire procedure will require at least two pints of fluid. No, it’s not a waste because the old fluid will corrode the internal parts of the servo, as well as other brake parts, and make rebuilding very difficult, maybe even impossible. The frequency of fluid changes probably depends more upon time than mileage: I suspect every 12 to 18 months is reasonable.

If your brake system has been well cared for, you will find the internal bores in the servo to be clean and smooth. If so, you can forget about honing the bores and concentrate on a very thorough cleaning before rebuilding.

The subject of bore honing is very controversial, especially for hydraulic systems built of aluminum such as the Girling units. If a cylinder bore is damaged due to corrosion attack or mechanical scratches, it must be smoothed out in order for the rubber seals to work without excessive wear. If the bore is bad, throw the unit out and try again! (Or, maybe get super machinist Tom Hall to make up a stainless steel sleeve.)

There are several approaches to honing and all of them have problems. Aluminum is soft and difficult to smooth out; the servo bores have small diameters that are difficult to hone and it’s difficult to thoroughly clean out the cylinders after honing. All in all, it’s a nightmare. If you can retain the bore concentricity while honing with a sandpaper-covered stick or a miniature honing stone, you will have beat the odds. Good Luck!

My point of this doomsday story is simple; Don’t hone a good bore! There is nothing harmful about the so called “glazed surface” on aluminum; feel it ? it’s smooth! There have been numerous tips on servo rebuilding, including the instructions provided with the rebuild kit. The kit directions are pretty good, but I’ve found a few modifications that have helped me.

• Good circlip pliers are necessary for removal of the output piston circlip. It’s a tight fit because the circlip is buried deep down the bore. I had to extend the tips on my pliers in order to fit.
• The spring tool for holding down the output piston while removing the circlip is worthless! It takes up space within the bore and interferes with the circlip pliers. It also prevents the piston spring from helping guide out the circlip.
• The new rubber strip that fits under the leather seal on the vacuum piston is often too large, causing the piston to bind and lock up the brakes. A great anti-theft feature, but not so convenient! Reuse the old seal if not damaged; otherwise carefully cut the new strip on one side so as to reduce its thickness.
• The leather seal and rubber strip will be saturated with brake fluid. Use clean paper towels or rags to blot up the excess fluid; keeping in contact with the strip for several hours will help.
• Silicone spray the leather and allow to soak into the seal. Be sure to use silicone (WD-40 is not silicone.) Allow silicone to soak in for a couple of hours and wipe off excess.

I use the “special servo grease” provided in the rebuild kit. Work it into the leather and coat the rubber strip. Also, wipe a thin coating over the vacuum cylinder bore. Rebuilding servos, master cylinders, etc. are big pains! Change your brake (and clutch!) fluid more often for less pain! Tiger and Alpine owners over time have experienced a variety of maladies centered on “stopping the beast”, i.e. brakes.

Contrary to common belief, the brakes on a Tiger are basically the same as those on an Alpine. Keep in mind that there are two body series for Tigers and Alpines. The Alpine came in Series IV and Series V and Tigers came as MK I and MK II. It is reasonably safe to say that at this time there are fewer than 5 percent of all Tigers and Alpines with their brakes in the best condition possible. Why so few? Several reasons: there are a number of brake service procedures that are required but almost never followed as most owners do not own service manuals or owners manuals which are needed for the necessary information. There are three items that are most often overlooked.

1. Brake fluid is hygroscopic; it absorbs water from the atmosphere and becomes contaminated resulting in internal corrosion (oxidizing) of all components, reduced braking power, and rapid brake fade under moderate use. The factory service procedure is to completely replace the fluid at one-year intervals. Start with the engine off, depress the brake pedal 3 x 5 times before bleeding to eliminate all vacuum in the booster. Bleed the rear (left wheel cylinder), first, then the left front, then the right front. Be sure to use only Girling amber or Castrol LMA (low moisture absorption).
2. To properly bleed the rear brakes, they must be adjusted out until the wheels will not turn by hand; this ensures the removal of all air that may be trapped in the cylinders. Brake pedal height is also determined by proper adjustment of the rear brakes, usually two clicks back from hard lock.
3. The brake servo air filter should be replaced every 6000 miles. Dirt in this filter or a blocked filter will result in a hard pedal or apparent lack of servo assist.

It should be noted that a Tiger or Alpine with brakes in proper condition can lock up all fours at 15-20 mph testing speed with standard 78-70 series tires and stock pads. For vehicles used in competition (auto-cross; short to medium length tracks) standard pads and shoes are fair, however, improved times may be had by using up-rated components.

As one additional point of interest, I recommend that each of the bleed screws be loosened and retightened at least once each 6 months and that they not be over tightened (5 lb. ft.). When servicing the rear brakes, the brake adjusters should be disassembled, cleaned, coated with never-seize compound and reassembled.

This does not apply to those later 1967 production Tigers and Alpines equipped with self-adjusting rear wheel cylinders. These should be checked at 6-month intervals for freedom of operation.

Still on brakes and in answer to a question from a fellow member, the blue-white smoke and heavy detonation or engine knock experienced after heavy braking is the result of brake fluid entering the engine through the servo to engine vacuum hose. This results from a failure of the servo piston rod bearing bush seal which forces fluid into the vacuum cylinder. If the failure has not occurred here, it has occurred at valve control piston either at the hi or lo pressure end which will allow fluid to enter the valve chest. In any event, the servo must be rebuilt.

Sunbeams from B9470001 thru B382991282 have small-chamber boosters that use Girling kit SP 2230; from B382001283, they have large-chamber boosters that use SP2228. If a new servo is needed due to scoring of a piston, #64049127 is for the small chamber, and #64049460 is for the large chamber. These numbers also apply to Alpine Series IV small and Series V large. To close this rap on brakes, let me say that the limiting factor on proper brakes in a Tiger or Alpine is tires, particularly on the Tiger. One can very easily overpower these tires with standard brakes. I’d suggest … well a lot of things, but for starters minimum size should be about 175/70?13 and not any smaller. Rim size and room in the wheel well determine the upper limits. LAT9 & 70 wheels and T/A 60’s (BFG) work nicely.

Editors note: A very reliable option to the brake servo is to bypass it with a brake line, replace the master cylinder with a smaller one and increase the diameter of the rear brake cylinders. Doug Jennings of Dayton Tiger can help you with all the parts.