Brakes and my son in africa

I’m visiting my son in west africa. He has a late '90s ('97?) nissan pick-up.

After replacing a brake line and bleeding the system, we drove to the airport to pick up my delayed luggage. With very little use of them, the front brakes began to seize up. We stopped and opened the bleed valves which caused the calipers to relax and were able to continue a little farther before the same thing happened again without using the brakes at all this time.

What would cause the front disc brakes to seize up?

Brake hoses maybe. After a decade or two, the inner lining can detach and act like a check valve allowing brake fluid to flow toward the brakes but not away. But it seems odd that they should both suddenly fail just after you’ve done brake work.

Problem is that despite my long history of innovatingly screwing up a multitude of simple mechanical tasks, I can’t imagine what you could do when replacing a brake line and bleeding brakes that would cause the front brakes to lock up. Maybe someone else has a thought.

Update - The next time it happened we disconnected the vacuum line to the booster and the brakes released. Thanks for any help.

So, maybe the brake booster is sticking? Plausible.

Sounds like a collapsed vacuum line, the inner hose lining can detach and acts as a non return valve.

Try replacing the vacuum hose to the servo first.

Look4info, thank you very much for posting back to let us know what you did to solve the problem.

Having heard that you disconnected the line to the booster canister, let me suggest that the real cause may be failinng springs in the master cylinder or perhaps sticking MC pistons.

Normally, the vacuum on the aft side of the booster diaphragm assists in applying the brakes however the return springs (combined with the line pressure) for the pistons of the MC allow the valves to open completely to allow relief of the pressure (fluid flowing back into the reservoir) completely even with the constant positive load of the booster. It sounds like in your case the springs may be too weak and allowing the booster to in essence keep the brakes slightly applied. That would also prevent total reliefof the hydraulic pressure in the brake lines, dragging the pads. If I’m right, then the dragging pads will heat up the cylinders and expansion of the fluid may be enough to do the rest.

I’m really straining my gray matter on this one, so please everyone don’t hesitate to chime in. If nothing else we can use this thread to explore reasons why the booster might be applying the brakes.

PostScript: the more I think about this is the more I wonder of it could be the spring in the booster canister that returns the diaphragm and allows the shaft in the MC to come back. Guys?

There must be a vacuum control valve inside the booster unit that is not working properly. This allows it to apply the brakes all by itself. On most cars, you can hear this valve open and close as the brakes are applied and released. These boosters are not field repairable. You must replace defective parts…

The valve you allude to is a check valve that allows vacuum to be applied to the diaphragm chamber but not pressure. It does not control the amount of vacuum applied. Vacuum through that valve to the fore booster chammber is the normal state of operation.

However, there is a spring in the booster that keeps the diaphragm from applying the brakes just from the vacuum, it offsets the diaphragm, and if this spring had somehow broken the scenerio described by the OP is theoretically possible.