Hydraulic brake system transmission

1 Mar

A late issue with brake system of the TD entails my investigation and reflection. So, I was very keen reading the article of Paul Ireland regarding the brake drum in issue 49. Paul’s article focuses on the drum and brake shoe, the active part of the braking system. Brake drum and shoes are the place where dynamic energy of the car dissipates into heat to slow down the vehicle when brake is applied.

Only one point to complete Paul’s interesting article and the main topic of my discussion, will be about hydraulic transmission from master cylinder to wheel cylinders. Though this system is often explained in many documents, there remains some untold facts regarding the detailed design.

As Paul says, the one pivot system implies a leading shoe and a trailing one with a self-servo effect on the leading one. The stress on the leading shoe being greater than the trailing one, the wear of the lining is greater. Then it was a current practice to install a longer lining on the leading shoe to compensate for wear.

To compensate for shoe wear out, it is necessary to have an adjustable system to maintain the shoes as close as possible to the drum when the pedal is released. Until the 1960s, two manual systems may be found on different car makes. Both systems need regular adjustment to maintain a short action gap on the pedal.

The first system is based on an adjustable stop for the shoe when returning to stand by position. This is the Bendix system (a schematic is shown below). As lining wear out takes place, the shoe stop is manually advanced to maintain the same gap between the lining and the drum. Therefore, the lining wear out is compensated by additional fluid in pistons. Then the pistons and the cups operate at different positions in the cylinders as the stop adjustment goes along. It might be necessary to refill the fluid reservoir with this first system. Also, as the cup operates at a new position at each adjustment, a cylinder leak may immediately follow a brake adjustment because of cylinder local pitting or corrosion.

When flushing the fluid with this first system it is better to release the adjustable stop to empty the wheel cylinders.

The second system is implemented on TDs and other MGs. This is the Lockheed system. The micro adjuster that we all know adds a variable distance between the piston and the shoe. Therefore, the lining wear out is compensated by a mechanical part. The fluid in the system remains constant and minimal. The cup/piston always operates at the bottom of the cylinder which remains almost empty when brake is released.

I can only see advantages for this Lockheed system.

Figure 1 Bendix system for wear out compensation

Figure 2 Lockheed system for wear out compensation

Apart from a messy job, bleeding the TD brake system might become a hassle on these cars. Bleeding goal is to completely fill the system with fluid. In other words, bleeding goal is to take out any air from the whole system. Unfortunately, bleeders are only on wheel cylinders and some high points remain with air bubbles trapped in these points.

High points for TDs are the front hoses, the relaxing loop at each rear wheel brake plate and the rear cylinders of the front wheels. The pipe between both front wheel cylinders is also a high point. A quick depress of the pedal might probably push air out of those small diameters high points in front of the fluid.

But the rear cylinders of the front wheels may lead to some difficulties. Since both front wheel cylinders are identical, the input and bleeder of the front one is on top but the rear one input is on the lower part. Air remains trapped in this cylinder. Furthermore, when pushing the fluid to the front cylinder, the air inside the rear cylinder is away from any turbulence of the bleeding operation. On the web, we can find recommendations to fill this rear cylinder with fresh fluid by the cylinder itself before fitting the rubber cup and the piston.

The DOT3 and DOT4 fluids are soon contaminated with moisture. Pour some water in a DOT4 jar. At first, they do not mix but the day after there is only one liquid phase.

When brake pedal is released the cups are pushed inside the cylinders by the retaining spring of the shoes, but despite the cup pushes the fluid back to the master cylinder, the inner wall of the aluminum cylinders remains covered by a very thin layer of fluid which soon gums up with moisture. Dust covers are less than perfect. I guess that nowadays rubber boots are much more efficient. Brake after brake, stop after stop, the gum layers accumulate and the piston gets sticky inside the cylinder requiring maintenance.

The Lockheed Service Manual recommends to renew the fluid at intervals of 18 month. But it also recommends to exchange cylinders or at least cleanup the cylinders and renew all rubber cups every 3 years. I bet that except for racers, no TD driver follows this recommendation nowadays. So, don’t be surprised when finding a stuck or leaky cylinder. Lockheed was probably aware of its system weaknesses. Aluminum cylinders are implemented for their moisture robustness without any corrosion to the cylinder wall. A simple cleanup operation allows to recover from a stuck cylinder.

DOT3 and 4 fluids are quite cheap and regular flushing of the system is the minimum operation to perform.

But low points of the system prevent the fluid to be totally flushed out. Draining the fluid is usually done by the bleeders. By definition, bleeders are located in upper points. Chances are that the fluid in rear wheel cylinders, and front cylinders of front wheels is never drained out (see Figure 3).

Figure 3 Front wheel cylinders arrangement

Flexible hoses are also a usual source of malfunction. These hoses should be flexible to allow for differential movement of wheels and chassis. But they should keep their fluid volume constant; hence a very strong material. Inside diameter of such a hose is about 4 mm and the total length of the three flexible hoses is about 90 cm on a TD. Let’s assume that its length is constant under pressure. A diameter dilatation of only 10 % represents a volume of 2400 mm3. This volume is equivalent to a displacement of 6 mm of the master cylinder piston and a free movement of 25 mm of the brake pedal. Adding the 13 mm free play of the pedal and we get a total of 38 mm gap before brake pressure is applied to the drums. Three times the normal free play! Due to flexible hoses aging and expansion, the pedal may feel spongy despite a correct bleeding of the system.

Soft pedal feeling may also be caused by flexible hoses when they become almost clogged by a thick fluid. Probably a mix of old brake fluid and the degradation of the hose inner material. When the pedal is depressed, the clogged hoses restrict the fluid flow giving this soft feeling on the pedal until all the linings contact the brake drums.

DOT 5 based on silicon, is not prone to moisture degradation. Switching to DOT5 for our vintage vehicles is a real improvement regarding maintenance. But this fluid is not compatible with DOT4 or DOT3. More precisely, they don’t mix (see picture). So, no solid material could clog the small holes or the hoses. But since DOT3 and 4 are denser than DOT5, DOT3 and DOT4 would accumulate in low points; cylinders which are never drained out.

Thus, there would still be the moisture issue. That’s why an efficient switching to DOT5 needs a complete flushing and cleaning of the system, tubing, cylinders, hoses. Don’t forget the brake switch.

Another good feature of the Lockheed rear wheel pistons. The rear wheel cylinder piston is actually split into two pistons. When acting on the handbrake lever, only the outer piston is pushed by the lever thus preventing any air suction passed the cup. This is clearly explained in the Lockheed Service Manual, figure 25.

The hydraulic brake system could appear very simple once installed but it is a great part of design engineering. And we could also examine the relative strength of all the springs in the system which are thoroughly chosen. Compared to cable transmission system it has the huge advantage of being self-balanced on all wheels. Filled with DOT5, the hydraulic system requires no maintenance. A lifetime of 10 to 20 years is claimed by fluid manufacturers. I’ve been running the same DOT5 fluid in a car for 13 years without any issue. And the TD will switch to DOT5 for next season.

Ed’s note: This article was sent to me by Laurent Castel. Laurent’s written English is very good and I have had to make only a couple of alterations. I hope that readers will find the article interesting.

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One Response to “Hydraulic brake system transmission”

  1. Gerry McCarthy 13. Mar, 2019 at 4:41 pm #

    Good article, what would I use to flush the system with, is there a special method to do this.I have found in the past, by accident, that raising the front when bleeding gave a harder pedal probably because I got more air out of the system as opposed to level bleeding
    Gerry McCarthy

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