Tech Articles

Vacuum reservoir

The brakes on Land Cruisers are vacuum assisted. This means that the truck has a brake booster, with a vacuum diaphragm. When the truck is running, a vacuum is created in the booster. Then, when you press the pedal, air is allowed in, and the air pressure activates the brakes. This makes the brakes much easier to apply.

But diesels don't exactly create their own vacuum, so Cruisers have a vacuum pump on the back of the alternator. In most cases the pump will provide enough vacuum, but to provide a more reliable brake system (especially for stop and go traffic), the trucks have a small tank, or vacuum reservoir. On my 42 it is located on the frame rail, under the driver, and it looks like a small muffler; on my 60 it is much smaller, located under the hood, on the firewall behind the alternator. The 60 location is much safer; on the 40s, it catches all the salt and spray from the front wheel, and thus they tend to rust out.

If there is a leak in the vacuum system - from the vacuum pump to the brake booster or anywhere in between - then you won't have power assistance, and the peddle will be hard to push. (Of course a hard pedal might indicate a different problem - like a bad brake booster.)

Testing

Here are a some tests from the chassis manual that will help determine if you have a problem with your booster or vacuum:

Check Air Tightness

1. Start the engine.
2. Stop the engine after running for 1 or 2 minutes.
3. Pump the brake pedal several times. If the pedal goes down deep the 1st time but gradually rises after the 2nd or 3rd times, it is in good condition.
4. If there is no change in pedal height when depressed the 2nd and successive times, it is defective.

Check Operation

1. With the engine stopped, pump the brake pedal several times with the same pressure. Ensure that the pedal height does not change.
2. Start the engine while the brake pedal is depressed. If the pedal goes down slightly at this time, it is in good condition. If there is no active change in pedal height, it is defective.

Check Air Tightness Under Load

1. With the engine running, depress the brake pedal. Then stop the engine while keeping the brake pedal depressed.
2. Hold depressed for 30 seconds. If the pedal height does not change, it is in good condition. If the pedal rises, it is defective.

Another way to test if your vacuum reservoir is leaking is to bypass it completely by running a hose directly from the vacuum pump to the brake booster. If the brakes are more or less normal with the hose, then you have a leak somewhere in your system. If not, then either your vacuum pump or booster are at fault. But depending on where you live, the odds are good it's the vacuum reservoir that's leaking. Of course you might want to install new hose clamps all around first, just in case.

Replacement

When the reservoir is leaking, you can try to patch it or replace it. Many people prefer just to leave it out of the system all together. Personally, as I do drive downtown now and then, I'd rather have the security of that extra vacuum. But instead of paying big bucks to Toyota (I've heard a new tank is about C$150), I'm going to replace the rusted reservoir on my 42 with a fire extinguisher bottle (suggested by Antti Rauramo).

The advantage, aside from superior strength and variety of sizes available, is that the fitting on the bottle is a standard 3/4" NPT, so plumping should be a relatively straight forward matter with a series of adapters. The one I have (thanks Mike!) is about 30 cm tall, and has a volume of about 1.5 litres (smaller than stock, but larger than the one on my 60). I've located it under the hood, beside the left hand battery. The location is nice because it's close to the brake booster (ie, less hose to worry about), and I can use the existing hard line from the vacuum pump on the other side of the engine bay.

Instead of just a simple tee fitting, with hose in and hose out, I did something a little more complicated. I wanted three fittings instead of two, in order to keep the vacuum sending unit in the system. Call me fussy, but I'd like that dash brake light to function fully. Of course the fitting on the sending unit is metric - 10 mm x 1.00 tapered, to be exact. Rather than bother with hunting down adapters, I simply used a lot of teflon tape, and screwed it into a 1/8" fitting (metric male will fit in a NPT female, but the reverse is not true). I also soldered a ground wire to the housing of the unit since my extinguisher bottle isn't properly grounded (the teflon tape too inhibits ground).

All said and done, I had a leak, audible when I shut the engine off. Surprisingly it wasn't at the metric/sae join, but at the pipe fittings (despite my generous use of teflon tape). So to be on the safe side, I RTVed all the joins. Since then, it's worked beautifully.