By Rob Robinette
Why would you need to
upgrade the master cylinder?
When you upgrade to larger brake calipers
your brake pedal movement may increase because more brake fluid has to be moved to power
the bigger (or more) pistons. Upgrading to the Mazda 929's master cylinder (MC) can bring
the pedal movement and feel back to stock (or better than stock). The stock master
cylinder is 15/16" in diameter, the 929's is 1" in diameter and about 1/4"
longer. That 1/16 of an inch larger piston pushes quite a bit more fluid. After I did this
mod my brake pedal was firmer and it required less pedal effort to lock the brakes. I'm
very happy with this upgrade.
The only difficult part of this mod is the
fabrication of two 12 inch lengths of brake line which can be done pretty easily if you have
a pipe cutter and pipe double-flare tool. You can always take the 929 master cylinder to a
brake shop and have them fabricate and install the brake lines. You can do everything else
and save some cash.
cylinder comparison. The rear brake line connector on the 929 MC is designed for
a flare fitting, not a banjo bolt like the RX-7 MC.
What You Need
A 93 Mazda 929 master cylinder part #
HG31-43-400G (ABS version) - I got mine from Jeff Haas Mazda for $170 (that's a 25%
discount for ordering from their 800 number: 800
233-2991). Pep boys wanted $214 for it. Mazda Competition Parts
will sell it for $135 if you are a qualified racer.
You also need a 929 MC metric flare nut, the MC
comes with the banjo bolt, but you need a flare nut for the rear MC brake line
Also pick up two 12 inch lengths of 3/16 inch brake
line ($4 auto parts store), a pipe
cutter ($7 Home Depot), pipe double flare tool ($40 Pep Boys, do
not use a single flare tool-it's not safe), 10mm flare nut wrench ($4
auto parts store--if you try to use a standard 10mm wrench you will round off the
Manual - Master Cylinder Removal/Installation
Removing the Master
Do not attempt this modification
yourself unless you are a very competent mechanic. Two short brake lines will have to be
fabricated and a single point failure in the brake system will result in the total loss of
brakes and possible damage/injury/death! A brake shop can fabricate the brake line and
hook it up to the new master cylinder.
Before you remove any of the brake
fittings, carefully clean all around them, so that dirt can't get into the system as the
fittings are opened.
Start by putting some rags below the master
cylinder (hydraulic fluid will bubble and peel your paint very quickly). I recommend
getting a pack of various size vacuum plugs for this purpose. I put plugs on each of the
two brake lines and one on the hydraulic clutch connection to prevent excess fluid
drippage. Use a 10mm flare nut wrench which is the SST (Special Service Tool) the
Workshop Manual calls for to loosen the brake lines that go into the double brass
fitting next to the filler cap (item C in the first diagram above). I highly recommend you
buy a $4 10mm flare nut wrench (the SST mentioned above). It has a wider jaw to keep you
from rounding off the nuts and its length will help you torque the brake fittings properly
(it's hard to do with a 5 inch long 10mm wrench). By disconnecting the brake at the brass
double fitting, very little fluid will drip out. I put vacuum hose caps on the brake lines
once I got them loose. Remove the two nuts that hold the MC to the brake vacuum chamber.
Remove the large vacuum line running to the vacuum chamber to relieve the vacuum pressure
and remove the MC. If you don't release the vacuum you will have trouble getting the MC
You'll have to swap the master cylinders'
reservoirs because the 929's doesn't have an hydraulic clutch connection. Remove the
Phillips head screw below the reservoir and pop off both reservoirs and then swap them.
Use a little hydraulic fluid on the rubber seals to make the installation of the reservoir
easier. Reinstall the Phillips head screw to secure the reservoir.
The 929 MC has a fitting for one of the two
brake lines on the wrong side (engine side, rather than towards the fender). The
brake line needs a metric flare nut, the other already has a banjo bolt with a
flare fitting installed.
The new front brake line
Both brake lines on the MC will have to be replaced.
I bought two prefabricated, 12 inch long 3/16 inch brake pipes, cut off one end,
removed the flare fittings, put on metric flare fittings, bent them over a 2 x 4 railing to get a nice 180 degree bend, cut it to fit with a
pipe cutter, put the other metric flare fitting on and double-flared the new
pipe. See Photo. See the Flare a Brake Line How To for
instructions on how to double flare a brake line. The rear line runs straight out
from the rear flare fitting a couple inches and bends down and under the MC, coming up to the
brass junction block. Make sure the lines don't rub the MC or its support. Connect the new brake lines and torque
the connectors to 113 to 190 inch pounds (10 to 15 foot pounds).
The front (right) and rear 929
master cylinder brake line fittings. Photo by Paul Winter
Mount the master cylinder in your car by
securing the two nuts and connecting the hydraulic clutch line but don't hook up the brake
lines yet. Screw the correct size bleeding kit fittings into the two line connectors at
the double brass pipe fitting and run the hose from the fittings up into the master
cylinder reservoir (the fluid loops around from the master cylinder's outputs back into
its reservoir). See photo above. Fill the MC reservoir with brake fluid and pump the brake
pedal until you don't see any more bubbles coming out of the hoses. It took me about 5
minutes of pumping. Remove the bleeding hoses one at a time and hook your brake line up
quickly so you'll limit the amount of fluid that runs out. Torque the brake lines to 113
to 190 inch pounds (10 to 15 foot pounds).
Follow this procedure to
bleed the master cylinder (very
important) to remove air from the master cylinder before pushing that air
through your ABS and brake system.
Bleed the brake system and do a pressure
check. I had very little air in the brake system, the MC bleeding worked. See the Brake Bleed How To.
My brake pedal is firmer now and there is less pedal travel than with the
stock MC. Be careful during your test drive, your brake vacuum will take about 30 seconds
to replenish itself. Good luck.
Adjusting the MC Piston to
Push Rod Clearance
I didn't do this because the piston
extension of the 929 MC was exactly the same as the extension of my stock MC and of course
I didn't have the three Special Service Tools (SST) to do it by the book. My brakes work
great without the adjustment. The RX-7 Workshop Manual describes the procedure on
page P-12. It shows three SST's being used. I scanned in the images from the manual as
best I could.
the MC Piston to Push Rod Clearance
More On the Master
A little history... A few months back, a
prototype Cooltech brake system was installed in Project RX-7. Our goal was to play around
with different systems and see which set up worked the best. Our first system consisted of
14x1.1" Metal Matrix Composite (MMC) rotors with Porsche/Brembo calipers (from rear
of 993 turbo). FYI, MMC rotors are approx. 70% lighter than traditional iron rotors. They
also dissipate heat up to 5 time as fast, according to Cooltech. Although much larger, and
probably far stiffer, than the stock 4 piston calipers, the Brembo caliper had smaller
pistons. This greatly reduced pedal travel (which was good), but reduced the clamping
force of the system (which was bad) at a given pedal pressure. In fact, with this set up,
the stock rear brakes would do more work than they should. Once ABS kicked in, it behaved
as an electronic proportioning valve, which somewhat masked the problem. Until that point,
the car was very loose under braking (needed more front brake).
Then we jumped on the other side of the
fence: larger-than-stock pistons from the "big red" caliper (even more enormous
caliper-from the front of a 993 turbo). We also increased the thickness of the MMC rotor
from 1.1" to 1.25". Just barely fit behind the airy spokes of the big 17"
TE37s. Around town, this set up was very nice. In fact, pedal travel was very close to
stock. Braking force, on the other hand, was exceptional. However, on the track, while
braking from high speed, weaknesses became apparent. It almost felt as if there was air in
the system-it took two quick pumps to get the characteristically firm pedal it had when
driven on the street. Repeated brake bleeding did not help.
Today, thanks to Mostly Mazda, a larger
master cylinder from a '93 929 was installed. Stock bore is 15/16", 929 bore is
1". Doesn't sound like much of a difference, but it's significant. RESULTS: Firm,
firm, firm pedal under any condition I can manage on the street. Ever drive a 911 turbo?
Same feel. About one inch of free play, but then it stops. You modulate more with pressure
and less with travel. The pedal lines up right next to the gas pedal (and the David B.'s
pedal adapters, of course)-- making heel/toe footwork very easy. Braking is very, very
strong. Braking also feels more "direct" and far less vague. You can even feel
the holes in the rotor scrape against the pads. Very very fun. With RA-1 race rubber on,
braking from high speed is absolutely incredible. Decelerates so hard that it repeatedly
skips my stock CD player. It never skips otherwise. Looking forward to trying it out on
the track with a prototype set of trick brake ducts that should be in next week. Should
have a better idea of overall f/r balance. So far, I can find no fault. I'll update the
Thanks to Jim O'Brian for the 929 MC