Brakes

Episode #831

January 4, 2025

Several years ago my 1999 Toyota Tacoma pickup truck brakes began to fade. The truck just did not have the braking power it used to.

My general mechanic did all the usual tests:

1. Pedal test. The pedal acted normally but required more effort than usual to brake.
2. Brake fluid level. The fluid was full and clear, though every couple of years I had to add a little fluid. This may indicate a very slow leak. There was no evidence, however, of leaks in the reservoir, master cylinder, brake lines or calipers.
3. Bleed the brakes. To clear air trapped in the brake lines a clear tube is attached to all four wheel bleeder valves. An assistant pumps the brakes and air is pushed out through the tube. No air was present and the brakes were still soft.
4. Inspect the brake pads and rotors. No glazing was found, only normal wear. There was still life on all the brakes.
5. Check the master cylinder. My mechanic replaced the master cylinder. When this did not solve the problem. He put the old cylinder back on.
6. Check the brake booster. As with the master cylinder the mechanic test-replaced the booster with no improvement, so put the old one back on.

My mechanic threw up his arms and gave up.

“The brakes are safe enough, they just need more effort. Just drive it and I will research what may be causing this problem.”

I drove the truck for 2 years, limiting my use of it because I frankly did not feel safe.

On a whim, I Googled brake shops in my town and found a 30 year old family business run by 3 generations of brake and alignment mechanics.

Precision Alignment & Brake did all the same brake tests and did observe glazing on the rear brakes. The last time the brakes were adjusted it was done incorrectly. A brake replacement job was recommended and installed. The softness still persisted. 

The old man in the shop recalled that this Toyota was equipped with a Load Sensing Proportioning Valve (LSPV). The LSPV is connected to the rear suspension via a linkage. As the weight of cargo in the truck bed increases the LSPV allows more hydraulic pressure to the rear brakes, improving stopping power under a heavy load. With a light load the LSPV reduces braking force to prevent brake locking under hard braking.

The mechanic adjusted the linkage of the LSPV and tightened the connections. Problem solved.

In my general mechanic’s defense, the LSPV was covered in road grime hiding the linkage and brake line connections, making it difficult to see the valve. A very slow brake fluid leak likely contributed to the road dirt sticking to the valve.

It just goes to show that you don’t know what you don’t know until you know it.

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