I have a pile of parts in a rented storage garage that someday will be a '70 TR6. It's winter now, too cold and snowy to get the TRs out and thus a good time to do some bench work on the brake system. I also volunteered to host a Buckeye Triumphs "Brake Tech Session" using parts from the '70 as well as parts members bring with them. This is photo opportunity too good to miss. It is also an opportunity to write a description of the TR250 & TR6 Brake System and how one amateur mechanic overhauled his system.
Before jumping in, a word or two about organization. The brake system has many components and at first glance might seem overwhelming since it's distributed all over the car. I've found that projects such as this are best dealt with on a top down basis. With this in mind, I've organized this into eleven separate notes, this theory & overview followed by ten notes dealing with the nitty gritty of overhauling and troubleshooting the system components:
Past experience has shown that as soon as one of these set of notes are put on the club website I get a flurry of emails with suggestions & corrections. Breaking into smaller sections as done here permits quicker and easier revisions to incorporate all the corrections and usually some really neat suggestions. Also, in this case, the parts of several notes are being deferred awaiting completion of the frame overhaul for the '70 s that the brake pipes and cables can be installed.
One last point before digging in: these notes describe what I did on my car for my personal use and are provided here for entertainment; they are not meant to be instructions for others to do maintenance on their vehicles.
Overview: The same brake system with only a few minor changes was used from the first TR250 manufactured in 1968 through the last TR6 manufactured in 1976. The system is hydraulically operated using disk brakes in the front and drum brakes in the rear. Separate hydraulic systems are used for the front and rear so that a failure of either the front or rear hydraulic system should allow a measure of braking through the other half the system. The system employs a single master cylinder with separate chambers and reservoirs for the front and rear. The master cylinder is designed such that under normal operation the front and rear have equal hydraulic pressure. The system is equipped with a Pressure Differential Warning Alarm (PDWA) device that senses a difference in pressure between the two parts that indicates a failure in half the system. The PDWA is equipped with an electrical switch that turns on the red BRAKE warning light on the dash when a failure is sensed. The system is equipped with a servo that amplifiers the force applied to the pedal resulting in a reduced pedal force required to operate the brakes. The system also has a cable mechanical arrangement to operate the rear brakes for the hand brake function.
The variations to the system are:
There is a subtle but interesting point relating to the two different piston sizes. A little further on we see that the front brakes have no springs to push the pistons back into the calipers whereas the rear brakes have springs that push the pistons back into the wheel cylinders. Also, the front caliper pistons are much larger than the rear wheel cylinder pistons. The net effect is that as the master cylinder primary piston is pushed by the pedal a small pressure develops in the front system that pushes the pistons against the pads and in turn, the pads against the rotors removing all slack from the front system. This pressure is probably too small to overcome the force of the rear shoe springs with the small rear cylinder pistons. Once the slack is out of the front brakes, pressure will build as required. It is at this point the secondary piston starts to move. The interesting point is that since the master cylinder secondary piston is slightly smaller than the primary piston, the secondary piston will move further than the primary piston once the all slack is removed from the front brakes.
Nearly all the piston movement in the master cylinder and the calipers as well as the rear cylinders occurs under fairly low pressure. If there is no air in the system, additional force will cause little motion but instead cause the force of the pads against the rotors and the shoes against the drums to increase resulting in increased braking force.
Several things about the system are really neat. The secondary piston has the pressure of the front system on one side and the rear system on the other. This means that the front and rear systems operate at the same pressure; otherwise the secondary piston would move in the direction of the lower pressure until the pressures equalize. (This neglects the effect of the springs pushing on the secondary piston. However, the spring forces are small, nearly equal and oppose, so they essentially cancel.) This same feature allows the system to self adjust to differences in the fluid required to the two halves. For example, if the rear brakes are out of adjustment, the secondary piston will have to move further to provide sufficient fluid to move the shoes against the drums. The primary piston will also move further (more pedal to apply brakes), but the pressure in the two halves will still be the same.
Now what happens if part of the system fails? Lets first assume a rear brake line ruptures. When the pedal is pushed the same operation as described above will happen except that the secondary piston and hence the primary piston and the pedal will not encounter much resistance until the secondary piston runs into the back of the cylinder. After that point the secondary piston can't move any more and the pressure can then build between the two pistons and in the front system. On the other hand, if the front system ruptures, the pedal and primary piston will not encounter resistance until the primary piston physically runs into the secondary piston and then moves it to the point that the rear shoes are against the drums.
The failure of either half the system will significantly increase the brake pedal motion and will at best provide barely adequate emergency braking. The car should not be driven until all brake problems are repaired.
The PDWA is an H shaped pipefitting made of brass. The front leg of the H provides the hydraulic path for the front brakes and the rear leg of the H provides the hydraulic path for the rear brakes. A small piston rides in the cross piece of the H and prevents fluid from flowing between the two sides. If the pressure is the same on each side of the H, the piston will not move. If the pressure is different between the two sides, the piston will move toward the lower pressure side. The PDWA is equipped with an electrical switch that operates if the piston moves off center. The switch and piston are shown in the right photo below. The switch plunger normally rides on the narrow part in the center of the piston. If the piston moves off center, a larger diameter part of the piston comes under the switch and pushes the plunger into the switch, operating the switch in the process. The operated switch will then turn on the BRAKE warning lamp on the dash.
The pipes (plumbing): The sketch below the shows the brake pipes. (This sketch was copied from the TRF TR250 catalogue and then "processed".) The master cylinder and PDWA are both mounted to the body. The fluid for the front brakes goes through a short pipe from the PDWA to a tee mounted on the frame and then through separate pipes along the frame to each front suspension tower. A short hose connects the pipe on each tower to a pipe on each front caliper assembly and then via that short pipe to the caliper. The drawing is a little misleading in that the routing of the pipe between the suspension towers is actually on the back side of the frame cross member rather on the top as implied from the drawing. The fluid for the rear brakes goes through a pipe from the PDWA along the inside of the frame member under the top cruciform plate to a tee mounted on the top of the left frame member. A hose connects from the tee to a pipe on the left suspension arm that runs to the left wheel cylinder. A pipe runs from the tee across the front of the rear suspension cross member to a fixture on the top of the right main frame member and then to the hose that connects to the pipe on the right suspension arm that connects to the right wheel cylinder.
Front Brakes: The right front suspension from my '70 TR6 is shown below. These have been off the car since the late '80s. The rotor surface would normally be smooth from usage. (Hopefully they'll look a little better after we've overhauled them.) The caliper is to the rear upper side of the axel. The fluid input pipe and bleed screw are on the upper back side of the caliper.
Now what happens if one pushes the pedal part way and the master cylinder piston has moved part way but hasn't reached the point where the pads and shoes are firmly against the rotors and drums --- the master cylinder piston is still easy to push because the pressure in the hydraulic system hasn't started to build? Will the diaphragm plate continue to move to the left forcing the left push rod further into the master cylinder? The answer is (fortunately) no.
Now, back to the case where you're holding the pedal part way down and have not yet encountered significant back force from the master cylinder piston. The pressure on the pedal side chamber will continue to push the diaphragm plate a very short distance till the valve to the atmosphere closes and then slightly further till the valve between the two chambers opens just long enough to allow the pressure on the two sides to adjust to exactly match the back force from the master cylinder; it then stops moving. This is the "automatically correcting the performance of a mechanism" from the definition. Slick!
Now what if:
The forces: Guess
we've beat the servo to death. But, before we leave the theory, let's try to
get a handle on the hydraulic pressure in the brake lines and the forces on
the shoes and pads. Lets assume you slam on the brakes with the engine
decelerating. We should get 250 pounds net force from the servo (high
depression in the manifold due to deceleration). Let's assume you can put 80
pounds pedal pressure, which translates to 308 pound force (amplified by the
3.85 pedal mechanical ratio). Hence, we get 308 pounds force on the primary
piston without the servo and 558 pounds with both pedal and servo.
master cylinder primary piston diameter is 0.81" so the cross sectional
area is ~0.52 square inches. Therefore,
pistons in the front calipers are 2 1/8 inch diameter so the cross-sectional
is ~3.5 square inches so the ~590/1070 psi hydraulic pressure produces a
force of ~2065/3745 pounds on each of the four pistons and also on the
associated pads against the rotors.
The rear cylinder pistons are 0.7 inches diameter giving ~0.38 squares inches cross section and a force of ~225/405 pounds on each of the four rear brake shoes due to the ~590/1070 psi hydraulic pressure. The brake shoes are really levers that pivot around the around the adjustor at the one end and are pushed by the wheel cylinder piston on the other end. The lever has the effect of increasing the force of the shoes on the drum somewhat --- maybe by a factor of two if the brakes are adjusted snug to the drums.
The measurements are plotted below. The curves are a little erratic; the system operation is probably more linear than indicated. The problem was that I had a difficult time holding the scale steady. The gauge topped out at 1000 psi, which is why the "With Servo" curve stopped at 1000 psi. The "Without Servo" stopped at about 110 pounds pedal pressure where I topped out. The computed values from above are plotted and are well within a reasonable error for all the assumptions and the "Rube Goldberg" test setup. I was a bit surprised with the servo operation. I expected the curve to kick in at a little higher pedal force and I expected the curve to be steeper initially. One thing is clear, using the brake pedal for a footrest is probably bad for the fuel efficiency.
I'm now ready to move from talking about the brakes to actually working on them.