Routine Maintenance of Fire Truck Hydraulic Brake Systems

By Doug Klink

The braking system on your fire truck can be one of several types; mechanical, all hydraulic, or straight air. Mechanical brakes and air brakes (especially for those of us who live in a dry climate as little moisture gets into the air tanks so they are basically trouble-free) require little care; Just keep them adjusted and sparingly grease any zerks. Hydraulic systems, however need some regular TLC to keep them operating correctly.

Smaller fire trucks often just have a master cylinder that pushes brake fluid to the wheel cylinders which push the brake shoes against the brake drums. The master cylinders are usually of the single type, they have one piston which applies the brakes to all four wheels. More modern systems use a dual diagonal system for safety, there are two pistons in the master cylinder, each one operates the brakes on two wheels on opposite corners of the truck or car. This has been required on your car since about the 1960’s. With the dual system if you have a broken line or hose, you don’t lose all you braking.

I had this happen on a 1954 Seagrave quint a friend was driving. We were driving several trucks to a parade, back when I lived in Kansas City. We drove down several large hills with no problems but as we came to a stop light in North KC, he suddenly turned on all the truck’s red lights and blew through a red stop light, coasting to a stop with the aid of the handbrake (did I mention they are important too?). Fortunately it was a β€œT-shaped” intersection coming into our street and little traffic was around. Turned out a previous owner had done an improper repair to the right rear brake, using a non hardened bolt to replace an anchor pin. The bolt broke allowing the brake shoe to rotate away from the wheel cylinder and letting the piston come out of the bore. This resulted in all the brake fluid running out of the open bore, hence no brakes.

For this reason many antique car owners replace the old single system with a dual system when they restore a car. This involves replacing the master cylinder and all the brake lines. I don’t usually replace the single system, I just maintain the original system properly. How do we do that?

First, I always pull the wheels on a new truck and make sure everything is OK. Usually the wheel bearings need grease anyway so it’s a good move. You can check for contamination on the shoes and scoring on the drums and make sure there are no broken parts such as springs. Also check the three hoses. Two in front, one in the rear. If they look cracked or the ends are super rusty, change them. You can get new ones at most good auto parts stores if you bring in the old ones. In rare cases the inside of the hoses can collapse and cause the brakes to drag, even though they look OK on the outside. If the shoes are worn, take them to a good brake shop to be relined, but be sure the shop uses a lining material with the same coefficient of friction. New lining materials are generally too aggressive.

Once you’re sure that all is well inside the drums, check all the brakes lines for rust and damage. We’re doing a brake job on our ’56 Seagrave pumper right now and had to replace most of the lines and hoses due to external rust. It’s also common to damage your brake lines trying to replace hoses with rusty ends. There are new kinds of line available now that are much softer than steel lines, corrosion resistant, and can be hand bent easily. NAPA has them and you can also get them over the internet.

Then take a look at the master cylinder and see if fluid is dripping out of the rubber boot on the back. If it is, you need a new one, it’s just a matter of time before it fails. We were able to get a new master cylinder and all 4 wheel cylinders for my ’56 Seagrave new from NAPA for about $250 total. They can’t look them up in their computer by make, you have to take the old parts in and they find them by casting number and shape. You can also send the master and wheel cylinders off to a shop to have them sleeved and rebuilt. I use Brake and Equipment Co. in Minneapolis if I’m going to do that. Google them for contact info. The bores are honed out and a very thin stainless steel sleeve is pressed into the bore. Voila, no more rust problems but you still should bleed the fluid regularly. It costs about twice as much to have them sleeved and rebuilt than buying new ones, but if yours are no longer available it’s a good option.

If you are replacing your master cylinder, it needs to be bench bled before being installed. Put it in a vise and run some tubing from the outlets into the reservoir. Fill the reservoir and pump the master cylinder until no bubbles come out of the lines. Then install. This simple step saves many problems later due to trapped air.

Once your system is sound, it’s time to address the lifeblood of the system, the brake fluid. Most manufacturers recommended a brake fluid flush every few years. Why? Brake fluid is hygroscopic; it absorbs moisture. The moisture settles in the lowest points of the system and causes rust in the bores of the wheel cylinders. It also lowers the boiling point of the brake fluid which can result in no brakes after several hard stops, especially with drum brakes. Some people switch to silicone brake fluid (DOT 5) as it doesn’t absorb moisture and has a higher boiling point then glycol (DOT 3 or 4). I’ve not had good luck with it, it can be hard to bleed the system due to trapped air and pedal feel can be mushy. It does also have the advantage of not damaging paint if you accidentally spill some onto a painted surface. Glycol fluid will so be careful.

Brake bleeding is covered in most auto restoration manuals and is quite simple. Suck all the brake fluid out of the reservoir with a turkey baster. Not the one from your kitchen. Fill with fresh DOT 4 fluid (you’ll need at least 2 quarts) as it has a slightly higher boiling point than DOT3. Starting with the wheel farthest from the master cylinder, hook up a hose to the brake bleeder and run it to an old clear liquor bottle. Save the liquor as you may need it later if things don’t go well. Open the bleeder (you may want to invest in a bleeder wrench if your fittings are hard to reach) and have an assistant push the pedal to the floor. Close the bleeder, have the assistant raise the pedal back up, and repeat until the fluid runs clear with no bubbles. Don’t let the reservoir run dry or you’ll get air in the system and have to start over. You’ll be amazed at how disgusting and black the fluid will probably be. If you do this every couple of years you’ll greatly extend the life of your brake system.

Power Boosters
Larger trucks will have some type of power booster to multiply the force of the foot pedal. They are usually vacuum operated. On mechanically braked trucks, they are just a vacuum cylinder that pulls on the brake rigging to add some force. They usually have a leather diaphragm, and putting a little neats-foot oil in through the vacuum fitting occasionally will keep the leather supple. This works best if you can pull the cylinder and move it around to distribute the neats-foot oil and let it soak for a few days. Neats-foot oil is available at farm stores. Neat is an old name for cattle, and the oil is made from the shin and foot bones of cattle, hence the name. There’s also a control valve which is inserted into the brake rigging so it can sense when the brakes are applied and allow vacuum into the vacuum cylinder. The control diaphragm in the control valve can go bad and cause a big vacuum leak but they are pretty simple and can usually be replaced easily if cracked.

The standard brake booster is called a Hydrovac. It’s basically another master cylinder but with a big (10” diameter or so) vacuum cylinder attached. The regular master cylinder pilots the control valve in the Hydrovac, so think
of the Hydrovac as a force multiplier. It senses when you push on the brake pedal and multiplies that force. How much? A 10” diameter piston has 78.5 square inches of area, and an engine at 15” of vacuum develops about 7.5 psi of vacuum so the vacuum piston can, in effect, put a whopping 588 pounds of force on the brake pedal. That’s a pretty manly push!

Hydrovacs were used on many vehicles, from military and fire trucks to forklifts to dock cranes and they are still used today. There are several places that rebuild them and you can usually buy an exchange 5 unit. If not you can send yours in to be rebuilt. There’s not much you can do to maintain them. They have a leather or rubber vacuum diaphragm or piston seal and a little neatsfoot oil will help the leather variety stay supple. For a rubber diaphragm a little ATF usually is a good lubricant. They have a bleeder valve and when bleeding a brake system that includes a Hydrovac I always bleed it first, and sometimes last too, just to make sure all the air is bled out of the brake system.

Hydrovac problems can be the cause of mysterious brake fluid consumption too. If the seals in the Hydrovac’s master cylinder start to leak, the fluid will make it’s way past the vacuum piston seal and get sucked into the engine. If your brake fluid reservoir keeps dropping but there are no visible leaks in the system it’s a good bet the brake fluid is being burned in your engine!

A good source for parts and rebuild services for Hydrovacs is:

Brake performance and safety can be enhanced and service life greatly increased by routine maintenance including regular flushing of the brake fluid, lubrication of contact points, and looking for warning signs such as brake fluid level decreasing faster than it should.

Silver trumpet – 2012 Vol. 2