Brake Question – 79 FJ40 - Need Help (1 Viewer)

[1979 FJ40ish]

During the restoration of my 79 FJ40 I did a completed rebuild of the brake system to include replacing the rear brake pads, brake cylinders, front disc brakes and installed a rebuilt brake booster/servo and a new AISIN dual master brake cylinder.

I’ve only test driven the FJ40 for about 50 miles over the last 3 or 4 months since it’s become “drivable”.

I’m confident the brake lines have been bled correctly, there are no leaks in the system and the brake fluid levels in the master cylinder reservoirs have not changed.

Here is the issue as I drive it around the neighborhood the first few uses of the brakes feel normal but after that the pedal sinks an inch or two closer to the floor with the same amount of foot pressure. Additionally, I’ve never been able to lock up the brakes as in an emergency stop.

I’m far from an expert but it seems to me that if there aren’t any leaks in the system then the problem must be in either with the brake booster or the master cylinder.

The check valve on the brake booster does work.

Am I correct in thinking that if the brake booster was failing the brake pedal would get harder to push?

Do the symptoms I’ve describe seem to be consistent with a faulty master cylinder?

Is there something else I’m overlooking that can be causing this problem?

I’d appreciate your thoughts.

 

[coonassjohn]

It sounds like possible air still in the system.

 

[redv8tbifj40]

In my experience, a failing booster will not give any assist and like you said, will give you a very hard brake pedal engine on or off. To test the master, try stepping on/releasing the brake pedal 3 or 4 times with the engine turned off. This will use up the vacuum assist from the booster. At this point you should have a firm brake pedal. Again with the engine shut off, apply firm but not hulk hogan pressure to the brake pedal and hold. Does the pedal stay solid or does it slowly drop towards the floor? If the pedal slowly drops to the floor under firm pressure, you have internal leakage in the master. If the brake pedal holds firm, the master is not the problem. As long as the brake pedal holds firm, keep the firm pressure applied and start the engine. Now the booster should work and cause the pedal to drop a little. Let me know what you find after trying above.

 

[redv8tbifj40]

I also wanted to ask if you are sure your rear brake shoes are properly adjusted? You should be able to hear a slight amount of drag from the shoes while spinning each rear wheel. If your rear brakes are too far out of adjustment, you will not be able to make a panic stop as the shoes have to move too far to take up the slack before the brakes start doing anything.

 

[fjc-man]

was the distance ( freeplay) between the master cylinder and the booster pushrod set correctly? You said both were new?

 

[65swb45]

Lots of good suggestions here. Looking for your response before adding my .02

 

[1979 FJ40ish]

Thank you all for your great comments.

I haven't started the cruiser in over a month. I don't want to change topics here but I can't start the cruiser without priming the carb after it sits for several days. I'm having it towed into a shop on Tuesday because of my concerns about the brakes but also because I want them to experience the starting issue first hand.

Having said that, I just tested the brake pedal and it seems firm and seems to hold but it will drop toward the floor if a bit more pressure is applied to the pedal.

Based on my measurements the free play on the push rod between the master cylinder and the booster was set at 0.32mm with a target range of 0.1 - 0.5mm.

I believe the rear brakes are set correctly but I'm going to have the shop recheck them to be sure.

Is it possible to have an air leak in the system without having a visible brake fluid leak?

Is it possible to have air trapped in the master cylinder? I bench bled the master cylinder and plugged the outlets but then it sat for several months before I finished and cruiser assembly and hooked up all the brake lines and added brake fluid and bled the system.

 

[reddog90]

Thank you all for your great comments.

I haven't started the cruiser in over a month. I don't want to change topics here but I can't start the cruiser without priming the carb after it sits for several days. I'm having it towed into a shop on Tuesday because of my concerns about the brakes but also because I want them to experience the starting issue first hand.

Having said that, I just tested the brake pedal and it seems firm and seems to hold but it will drop toward the floor if a bit more pressure is applied to the pedal.

Based on my measurements the free play on the push rod between the master cylinder and the booster was set at 0.32mm with a target range of 0.1 - 0.5mm.

I believe the rear brakes are set correctly but I'm going to have the shop recheck them to be sure.

Is it possible to have an air leak in the system without having a visible brake fluid leak?

Is it possible to have air trapped in the master cylinder? I bench bled the master cylinder and plugged the outlets but then it sat for several months before I finished and cruiser assembly and hooked up all the brake lines and added brake fluid and bled the system.

Any updates on your brakes?

 

[1979 FJ40ish]

Unfortunately, no! It’s still in the shop. It is a busy independent shop and since it is not a daily driver, they will only work on it when they have time.

The owner told me his neighbor brought in a FJ40 with the exact symptoms. He said that by the time they were finisher virtually everything brake related had been replaced either by the owner or the shop. The shop owner said they spent a lot of time trying the find/fix the problem but could not fix it.

He said his neighbor then took it to a local Toyota dealership. They had it for several weeks and could not fix the problem.

His neighbor is unable to drive his cruiser.

The whole situation is baffling to me since in the scheme of things the brake system seems to be pretty straight forward.

I’m hoping my issue turns out to be something simple.

I have been meaning to ask a follow-up question. Will the brake booster check valve work correctly if it is not in a vertical position?

 

[coonassjohn]

I would rescue the cruiser and fix it yourself. Any shop that is to busy to work on your car doesn't need to waste your time. Brakes are not rocket science, work by the numbers and it will be fine.

 

[coonassjohn]

This may help locate the problem.