Installing Servo-boosted Brakes

Converting 'To' or 'From' Servo-Boosted Brakes
As of: 14 March 2002

"The 1975-76 and 1977-1980 models used two different master cylinders that were servo-boosted to provide power brakes. These servo-boosted models did not produce more braking power, they simply required less effort at the brake pedal. Converting a 1974 or earlier car to the later dual circuit servo-boosted master cylinder is difficult due to the mounting flange having been turned 90 degrees. The fitting of a modified later pedal box (the pattern of the pedal box mounting holes on the sheet metal flange is different) that matches this Master Cylinder flange pattern will solve this problem, but be advised that you will also need the later version pedals that were designed to be used with it or the clutch pedal will not depress far enough to disengage the clutch!"
- Steve S., Virginia

Well, that sounds simple enough.

And, right now, sitting side-by-side in my garage are 2 cars I'm building: a 1970 MGB GT for my wife, and a 1979 MGB that I'm converting to V8 power for me. So, switching brakes from one car to another appears pretty easy on the surface.

I want to install power brakes in Jerri's 1970 GT. She won’t be driving anywhere near its limits; heck, if she ever does actually drive it, she'll only go to the grocery store, and power brakes will make it easier for her to press the brake pedal.

And, I want to be able to "feel" the brakes in my V8 car. There's lots happening to that car's brake system (4-pot Rover SD-1 calipers, OE MGB GT rotors, MGC rear brakes, proportioning valve, etc), so I need to go back to an earlier non-servo-boosted dual circuit brake system.

This article will attempt to cover both conversions: from non-servo to servo-boosted in Jerri's '70 GT; &, from servo-boosted to non-servo in my '79. I'll try to keep them separate.

1970 MGB GT	1979 MGB V8
Steve’s short description is a bit simplistic when one is going from non-servo-boosted brakes to servo-boosted brakes. Yes, the "mounting flange having been turned 90 degrees" does pose a problem; however, there are other more serious ones as evidenced by the photos below.	Steve's description doesn't really cover this facet of the conversion; however, converting my '79's brakes to non-servo-boosted is pretty straight forward as the below explanation details. Actually, its pretty much "old brakes out, new brakes in."

The left photo is of the firewall area in Jerri’s 1970 Glacier White MGB GT; the right one is of my Pageant Blue MGB roadster.

The holes through which the pedals go are different requiring that I cut Jerri’s firewall to match my ’79 firewall. (I'll try to post a template later.)

Though the hole in the '79 servo-boosted car's firewall is larger than needed for non-servo pedals, there's no problem keeping it that size. Actually, the larger hole makes installing pedals a bit easier as you only have to remove the clutch pedal, & it can be installed from the engine compartment.

Additionally, because the master cylinders are located on the servo-boosted brake system with the clutch master cylinder back near the firewall bulkhead and the brake master cylinder out on the front end of the servo, the brake and clutch slave cylinder pipes must be completely replaced.

There's no pressure failure switch and body assembly in the ’79 servo-boosted brake system so one must be added from the ’70 GT's system along with the '70's brake & clutch slave cylinder lines. The location may be different than where it was in the '70 GT because I'm using RV8 headers. (More to come on that issue later.)

So, the items actually needed to complete the conversion of the '70 GT to servo-boosted brakes include:

Master cylinders and servo in original pedal box with pedals from servo-boosted car.
Clutch slave cylinder line from servo-boosted car, plus
Engine compartment brake lines from servo-boosted car that run from the brake master cylinder to the front wheels as well as the brake lines that run to the connector under the passenger seat.

For the ’79 car that’s being converted to non-boosted brakes one needs:

Complete master cylinders in their original pedal box, complete with pedals from non-servo-boosted car, plus
Clutch slave cylinder line from non-servo-boosted car, plus
Engine compartment brake lines from non-servo-boosted car that run from the brake master cylinder to the front wheels as well as the brake lines that run to the connector under the passenger seat from non-servo-boosted car, along with both the Pressure Failure Switch and body.

There will also be some minor reshaping of the '70 engine compartment brake lines when putting them in the '79, and at least one hole - for the pressure failure switch and body - must be drilled in the '79's inner fender.

NOTE: To remove or install engine compartment servo-boosted brake lines, the hood must be removed & the hinges put in the closed position.

Once everything has been removed from the ’79 car, it's time to fit the system to the ’70 GT. First, enlarge the hole in the firewall as per the template shown in a previous photo.

Then, the later brake lines from the ’79 car must be retrofitted to the ’70 GT.

The previous photos show the brake lines when they were initially being test fitted. All the fittings & their mounting holes are shown with the exception of a sheet metal screw needed to secure the driver side brake line to the inner fender (it may have originally been there; we filled many of the holes we had thought wouldn’t be needed when we painted the engine compartment.)

At the same time, I test fitted the non-servo brake system to the ’79 car's firewall, attaching the pedals at the same time. Everything fit with the exception of one hole in the forward portion of the pedal box a minor problem which was solved by elongating the hole in the pedal box itself.

Additionally, sheet metal holes were drilled in the firewall to attach the pedal box cover. Originally, the screws holding the pedal box cover to the firewall were 10/32 blind machine screws. There's a lot of pressure on the pedal box cover as the pedal presses against the brake light switch mounted upon it. So, I'll need to modify the way in which the cover is attached to the firewall/pedal box to withstand that pressure. (More to come on that later.)

Next, the brake lines & pressure failure switch and body from the ’70 GT must be retrofitted to the ’79 car.

The pedal box/booster/master cylinders from the '79 car are now installed in the '70 car.

There were some special considerations to making that happen:

First, the pedal box itself had to be test fitted. That entailed drilling 3 new holes in the firewall on the fender side of the pedal box for bolts to go into the captive fittings in the side of the box closest to the fender.

Second, 2 holes had to be drilled in the firewall to accept non-captive bolts on the side of the pedal box nearest the heater. Underneath the firewall, a brace had to be cut away to allow nuts to fit over those 2 bolts.

This photo shows the brake lines from the '79 car as they are fitted to the '70 car and run over the firewall blanking plate to the passenger front caliper and the rear brakes.

That, however, presents a bit of a problem. Running the '70 car's lines in the '79 car was easy: take them out of one car &, after some minor bending, put them in this car. Then, the brake lines and pressure failure switch plus its body from the '70 GT must be retroffited to the '79 car. Piece of cake!

However, the way the pipes are attached to the earlier non-servo car is different than the way pipes are attached to later servo-boosted cars. Whereas the earlier cars use clips that are attached permanently to the body with welds, the later cars use clips that are attached to the body with screws. A trip to my MG Graveyard netted lots of single and dual pipe clips from servo-boosted cars.

The photo shows the brake lines running along the driver-side inner fender much like they are in non-servo cars. There's one minor difference in my car, however: since I'm using RV8 headers, the pressure failure switch and body had to be mounted higher on the inner fender than normal. That entailed some bending of the pipes.

When running the lines to the front calipers, make sure they don't bind or rub against the steering column where they pass between in and the body. Also, when you attach the passenger front line to the back of the suspension beam, make certain it angles across the beam exactly as it did in the donor car.

Another miscellaneous item I had to get from the non-servo-boosted car was the wiring loom for the pressure failure switch and body. The plug is different than the one on a servo-boosted car.

So, additional brake system items needed from the non-servo donor car are:

    1. Various single & dual
         pipe clips, and
    2. Wiring loom for
         pressure failure switch
         and body.

The clutch slave cylinder pipe is also an easy switch. However, it's larger in diameter than any of the brake pipes. So, when removing it from the donor non-servo car, its important to keep any clips that went with it, especially the dual brake/clutch pipe clip.

RETURN
TO 1970 MGB GT