Converting 'To' or 'From' Servo-Boosted Brakes
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
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
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
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
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
- 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
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
Once everything has been removed from the ’79 car, it's time to fit the
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
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
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.
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.
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
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
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
So, additional brake
system items needed from the non-servo donor car are:
single & dual
pipe clips, and
2. Wiring loom for
pressure failure switch
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.