MGB Power Brake Upgrade: Fitting a Wilwood Master Cylinder

MGB Power Brake Upgrade: Fitting a Wilwood Master Cylinder

as published in British V8 Newsletter, Volume XIV Issue 3, December 2006

by: Larry Shimp

(Editor's note: please make sure you read and fully understand the disclaimer at the end of this article. Brake system performance is absolutely critical to the safe operation of any car. Modifications that suit one owner or application may be very dangerous in a slightly different application.)

I have Wilwood Dynalite 4 piston calipers in the front and Merkur disk brakes in the rear of my car. I've been running the original MGB power brake master cylinder, but the brakes were always mushy with a long pedal travel. The problem is that the caliper fluid volume is considerably more than that of the original MGB brake system, and the 3/4 inch master cylinder needs a long stroke to displace enough fluid to stop the car. One suggestion is to use Wilwood 2 pound check valves in the brake lines to keep the pads in light contact with the rotors at all times and so decrease the needed piston travel. In my opinion, a better solution is to install a bigger bore master cylinder.

In carrying out an Internet search I found that Wilwood makes a 1 inch bore, dual reservoir master cylinder with the same flange bolt pattern as the MGB cylinder, and it only costs $120 (from Summit, part number WIL-260-7563) which is less than a new MGB master cylinder. If you decide to change your master cylinder, make sure to first read the instructions that come with it and be sure to follow all the manufacturer's instructions and recommendations.

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I got the Wilwood master cylinder and confirmed that the push-rod end of the housing fit the ID of the power booster perfectly. If it were too small there would be a vacuum leak, and if it were too big the master cylinder couldn't be installed. I assembled the master cylinder with a bit of silicon sealer in the contact area to assure a vacuum-tight seal. The distance from the flange face to the brake piston was identical to that of the MGB cylinder, and the total brake stroke was also identical, so the brake pushrod needed no modifications. All that was necessary to fit the new master cylinder was to turn the booster 90 degrees because the Wilwood cylinder has a vertical mounting flange whereas the MGB cylinder has a horizontal flange.

Removing the booster mounting nuts to rotate the booster can be difficult. The most trouble is with the nut at the lower right by the brake pedal. This is best removed from under the dashboard using a socket and 1/4 inch ratchet. Due to limited clearance, the wrench can only ratchet one click at a time, and only if a high quality ratchet driver is used that has minimal free play and fine ratchet teeth, I used a Sears Craftsman driver, but a cheaper one I tried first would not work. (Editor's note: from what I've seen, Sears 3/8" ratchets come with either 24, 36, or 45-tooth mechanisms. My favorite ratchet is Sears' simple, classic 45-tooth non-quick release "banjo-style" ratchet. However, I've seen other brands with as many as 72 teeth...) The same socket and driver works very well on the left lower mounting nut. For the upper mounting nuts, an open end or 12-point box end wrench works. (A 6-point box wrench is not satisfactory.) The good news is that the pedal box does not have to be removed.

To connect the two front brake lines, I used a "T" fitting (like the kind normally found at the rear-axle), screwed into the master cylinder using a threaded (Earl's brand) adapter from Summit.

The threads in the T-fitting and master cylinder are 3/8" 24 thread-per-inch, and this corresponds to an AN #3 (military spec) fitting. It is important that the ends of the AN fitting be ground down so that they do not contact any of the internal seats in the T or master cylinder; sealing is by the copper washers used on the MGB brake hose that normally goes on the T. Buy at least 3 new copper washers to allow proper rotational indexing of the T fitting. The adapter must be steel for strength. It is possible to buy a brass compression union at a hardware store that will fit, but this is too weak and is potentially dangerous!

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As an alternative to the T fitting, the pressure differential switch from a non-power MGB dual brake system can be used, but these are somewhat scarce and new ones cost over $300. If using the MGB switch/adapter, make sure it does not have an internal leak. This leak will show up as fluid loss through the pressure differential switch. The external leak can be fixed by replacing the switch with a bolt and washer, but the internal leak between the front and rear brake circuits will remain and will defeat the safety advantage of the dual master cylinder. Rebuild kits for these are available, and this is the proper way to fix it.

The brake line compression nuts that go into the old master cylinder will not fit the T or the new master cylinder. As was explained above, all the new nuts need to be 3/8" 24 thread-per-inch. These are readily available at any auto parts store by buying a short length of brake tubing with the appropriate fittings. Take along the T-fitting to be sure of the size. You can also use spare MGB tube nuts, but many of these are not threaded all the way to the end, and fully threaded fittings are needed for the T and master cylinder. The lines that go in the T have to have a double flare. (Buy a double flare tool to do it right.) A double flare also works for the master cylinder.

It is important to connect the front brakes to the rear fitting on the master cylinder. The rear chamber is the primary chamber, with the piston connected directly to the brake push-rod. The front chamber is actuated by the fluid in the rear chamber, or, in the case of a leak, by the rear piston pushing on the front piston. Note: before connecting the lines, be sure to remove the check valves from the master cylinder following the instructions that come with it.

In my view, the only installation deficiency is that I was not able to fit the pushrod seal. The seal is designed to prevent vacuum in the booster from sucking the fluid out of the master cylinder in the case of a piston seal leak. But leaving the pushrod seal off was acceptable to me because it is only a back-up system. In any case, the front fluid reservoir would not be affected. However, this is only my opinion. If you have any doubts, do not make this conversion.

Bleeding the Wilwood master cylinder is extremely easy because they provide bleed valves right on the master cylinder body (as shown in the photo); one for each brake chamber.

The conversion was not difficult and it made a huge improvement in brake feel. The pedal stroke is shortened by about 60 percent, and the pedal is now firm.

Note: in the picture, my brake lines appear to be copper, but they are not. These are the British brake lines that contain some copper in their alloy for corrosion resistance. Copper lines should never be used because they will become brittle over time and can break!

Disclaimer: This page was researched and written by Larry Shimp. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk. Please especially note that this article suggests a deliberate compromise between (shorter, firmer) brake pedal travel and (possibly reduced maximum) braking force as can be accomplished by increasing piston diameter in the master cylinder. Although Larry reports excellent results in his particular application, this compromise may be ill-advised for your application. For example in the event of a failure in the power brake booster, you might find that the amount of pedal effort required to stop the car is higher than you expected. (Incidentally, Larry has tested this on his car. Your car may be different.) There could be other un-foreseen safety problems. For further information we recommend "Brake Systems - OEM and Racing Brake Technology" by Mike Mavrigian and Larry Carleys and "Brake Handbook" by Fred Puhn.

Brakes are critically important safety equipment. If you're uncomfortable working on brake components take the work to a qualified professional.

Photos by Larry Shimp. All rights reserved.

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