[Jack Beck]

This winter we replaced the puck seals and bled the brake system. Everything worked fine while the weather was cold here in Wisconsin and the airplane rolls well out of the hangar. However after the last two flights, we felt the brakes dragging a bit when pushed it back in the hangar. We took off wheel pants to check the brake assembly and again put silicone on the pins. It looked normal and worked fine. This morning we pulled it out onto a sunny ramp and it again rolled without dragging. After about an hour in 80 degree heat just sitting without any brake application, pushing it back into the hangar resulted in some drag again. Anyone have an idea on what might be causing this?

 

[Sig600]

This winter we replaced the puck seals and bled the brake system. Everything worked fine while the weather was cold here in Wisconsin and the airplane rolls well out of the hangar. However after the last two flights, we felt the brakes dragging a bit when pushed it back in the hangar. We took off wheel pants to check the brake assembly and again put silicone on the pins. It looked normal and worked fine. This morning we pulled it out onto a sunny ramp and it again rolled without dragging. After about an hour in 80 degree heat just sitting without any brake application, pushing it back into the hangar resulted in some drag again. Anyone have an idea on what might be causing this?

Fluid heating up and expanding in the lines is my guess. For some reason it's not just moving back into the reservoir.

What are you using for brake fluid?

 

[Jack Beck]

Fluid heating up and expanding in the lines is my guess. For some reason it's not just moving back into the reservoir.

What are you using for brake fluid?

I was thinking the same thing just assumed that the expansion would go to the reservoir and overflow if necessary. Guess we could have a look at the reservoir and see if something obvious was going on to allow expansion of the fluid there.
We are using MIL-H-5606.

 

[Walt]

Make sure the brake pedals are not sticking, if the master cylinders don't fully retract they will not release the pressure.

 

[RVbySDI]

It has been 5 years now since I installed my brakes but I seem to recall there was someone selling a spring mechanism that I installed on my brake pedals to eliminate this type of problem. Does anyone know what this set up consisted of and if my memory is correct concerning someone on this forum selling a return spring mechanism for the brake pedals?

In addition to the springs I replaced the two bolts that acted as the hing for the brake pedals with one long bolt. This allowed for a smoother hinging action with the brake pedal.

 

[BobTurner]

Bad physics

Fluid heating up and expanding in the lines is my guess. ?

Everyone gets this one wrong. When it warms up, the steel brake cylinder and aluminum lines expand a lot more than the fluid. Pressure should go down, not up.

 

[Jack Beck]

I wonder if the answer is found in putting Walt and Bob's responses together. If the airplane was pulled from the cool hangar without any indication of drag and only began to drag after warming in the sun without the brake pedals having been touched, then perhaps the brake pedals are engaging the master cylinder slightly in both cases; but only when the disk and attending assembly are heated does interference and drag occur in the systems. Sounds like I need to take a closer look at the position of the pedals and master cylinders.

 

[dmaib]

I had this problem during Phase 1 and it proved to be brake pedals that were not retracting completely and keeping the master cylinder slightly engaged. Specifically, it was the co-pilot brake pedals on my airplane. I flew from the right seat from time to time during phase 1 and finally realized that the problem always followed a right seat flight. Very small, and I mean VERY small amount of pedal movement will cause this. I corrected mine by making sure the through bolt on the pedals was absolutely free. Never had the need to install the springs.

 

[Sig600]

Everyone gets this one wrong. When it warms up, the steel brake cylinder and aluminum lines expand a lot more than the fluid. Pressure should go down, not up.

Coefficient of thermal expansion for 5606 brake fluid is .00046. Aluminum is less than half that. Imagine what your airplane would look like if aluminum grew 4.6% in size with a 100 degree increase in temp.

Sticky brake actuator is a good start. Somewhere on this board there are the part numbers for some springs you can buy online that make absolutely perfect return springs.

 

[BobTurner]

Coefficient of thermal expansion for 5606 brake fluid is .00046. Aluminum is less than half that. Imagine what your airplane would look like if aluminum grew 4.6% in size with a 100 degree increase in temp.

Sticky brake actuator is a good start. Somewhere on this board there are the part numbers for some springs you can buy online that make absolutely perfect return springs.

You are absolutely correct. I was thinking of the "steel cables in an aluminum airframe" problem, and just assumed the oil had a low expansion coefficient. Thanks for the correction.
I agree with the others: make certain there is no drag on the brake pedal pivots.

 

[Jack Beck]

Just did a series of tests on the airplane and confirmed that it is the brake pedal/master cylinder that are causing the problem. It simply manifested itself more with the addition of new puck seals, new brake linings, and the arrival of warm weather in Wisconsin. Looks like we will give the long bolt and springs a try. Thanks everyone for maturing my thinking on this one.