Brake Pedal Issues

[ryan00davis]

Hi Everybody,

I've been having some issues with my brake pedal lately. after particularly long and hot drives (stop and go being the worst of it), I have a problem where pedal pressure doesn't return all the way.
The return spring will fully retract the pedal, but the top centimeter (in the mild case) or few (in the more extreme versions i've noticed), when you push on the pedal, you are just pushing against the spring, and after that initial travel, the hydrologic forces are engaged. Once you've hit the hydrologics, the pedal feels fairly normal, so I don't really think the problem is just air at this point.

If you let the car sit for a while, the problem will go away. So I'm confident in what the good version should feel like (cold car, or car with <30 mins or so of driving on a warm day) vs what it feels like when there is a problem.

I did a full brake system flush thinking that the problem might be air in the system, even though it didn't seem like what I think air in the system would feel like. Sure enough there was a bunch of air in the system (I recently bought the car used), but that didn't fix the problem, once all the air was out, after a long drive, the problem was back.

My leading theory (proposed by a friend of mine) is that the valve between the reservoir and the master cylinder is faulty. This means that the system isn't able to refill itself with fluid from the reservoir. The brake light switch limits travel on the return spring, I tried backing this way out, and it didn't seem to solve the problem after a long drive today. So my current best theory is to replace the master cylinder. A potentially related issue is that when I've noticed this issue, the rear brakes will also drag a small amount. At low speeds on a small hill, the car wont roll. Based on sounds. I believe it is a rear that is dragging.

I've been contemplating switching to a smaller master cylinder anyway, but am back and forth on going all the way to the willwood pedal box kit vs just getting the nissan master cylinder (I would also appreciate a smaller clutch master cylinder that would come with the wilwood pedal kit).

Anybody ever seen an issue like this? Any theories other than a faulty master cylinder? The fact that the master cylinder is in front and the engine is in back makes me hesitant to blame thermals, but it really does seem to be related based off on anecdotal evidence. A short drive from cold shows no issues. If it matters, I have the wilwood brakes, but standards pedals.

Thanks for any insight. -Ryan

[Brad Smith]

You still have brake lines running to the rear wheels. Trace the lines closely in relation to your exhaust setup, AND in relation to any coolant tubes.

[ryan00davis]

Hey Brad,
Yeah, definitely agree that the brake fluid and brakes could get hot, my mild disbelief was that the master cylinder itself would get significantly hot, especially compared to the setup it was designed for which is the master cylinder in the same area as the engine.

[Brad Smith]

If your lines to the rear are touching the tubes carrying coolant to and from the radiator, they are going to leech heat like nobody's business, ant it will carry all the way to the master...by the way, at a relaxed position, your brake pedal should have a little free play in it. Meaning the rod into the master should be at the end of it's travel, and the pedal should still lift a touch with your toe. If it isn't, you could be dragging your brakes constantly. (which would also cause heat buildup)

[ryan00davis]

What you're saying makes sense, but I would think that if my problems should be happening at two different times then.
When my pedal seemed to drop significantly before actually engaging the brakes, that was when the brakes were dragging. From what you're describing, I would expect my brakes to be dragging when my pedal didn't have enough room to fully release, so I should feel very high pedal pressure immediately at the top of travel.

[AZPete]

Before you start throwing money at it, check the length of the pedal push rod and a restriction in any of the brake lines. It could be that fluid is not returning quickly to the master cylinder quickly after each brake application. In a short trip you may not be getting enough residual pressure to cause a problem but in a long trip the residual pressure can build up to cause the brakes to drag. If the push rod is too long it could be restricting the back flow. Likewise, a restriction in a brake line, like a dent or kink, prevents a full back flow. After a long drive feel the discs for heat and you might locate one wheel, 2 wheels or all 4, which would tell you which is dragging hence which brake line to check. To confirm, on a wheel with a hot disc crack the bleeder screw to see if fluid squirts out and if the wheel then turns freely, which means you've released the residual pressur. If all 4 brakes are dragging, then I'd suspect the pedal push rod.

[ryan00davis]

Ah yes, I hadn't thought about the fact that the pressure to apply the brakes is a lot more than the pressure to release the brakes, so a pinched line would make perfect sense for this. Thanks for the guidance. I'll update when I get a chance to trace everything through, and check for heat issues at the same time.
Thanks -Ryan