Parking Brake Valve trouble potential

[SierraYankee]

Dear Skymaster pilots and mechanics;
what are the possible troubles a malfunctioning parking brake valve (the entire unit) could cause ? This is for a P337H, SN P33700305
Could a leaking valve cause brake pressure loss in both L & R sides ?
Could a 'broken' valve cause air to entrain into hydraulic lines, or oil leaks.

Any conceptual thoughts would be of help.

Thanks in advance

[mshac]

My understanding is the parking brake valve closes the brake line while pressure is held on the brakes, allowing the brakes to stay "set", with the pressure remaining in the line.

A failed parking brake valve would potentially keep you from setting the parking brake, but unless its also leaking fluid, I don't see how it could affect your brakes.

Attached are a few relevant pages from the P337H service manual:

[SierraYankee]

Thanks for the informative reply and relevant pages.

Can the parking brake valve (if physically damaged) cause air to be entrained in the brake lines ?
My brakes keep losing pressure and oil, and every 4th-5th landing the master cylinders need to be replenished. No oil on the floor or belly (unless it is squirting out on the runway during landings).

[mshac]

It certainly could. Have you removed the cover panel and inspected the valve for leaks? That brake fluid is going somewhere. You may find your sound deadening fiberglass near the valve is saturated with 5606.

[SierraYankee]

No oil leaks evident from the brake valve

[mshac]

5606 is red out of the can, but if exposed to air and light it will turn clear. I would take another hard look a the entire brake system. Any fluid could be your leaking 5606, and it may not be obviously red.

[SierraYankee]

Will take another hard look.
The master cylinders and calipers were overhauled, also the long actuator rods under brake pedals were bent. The A&P "straightened" them, but impossible to be accurate unless turned on a lathe. Same issue still.
Appreciate all the insight.

[wslade2]

I went through brakes nightmare couple years ago.

A definite you report is loosing fluid. So you have a leak somewhere. In my experience the leak wasn't obvious. Look for telltale gummy sticky drying 5606. You may not actually see a red drip. It is a high pressure system and doesn't take much a leak or ooze to feel soft. Unless it is obvious dripping you're going to have to count on feeling that tacky dry 5606 holding dirt to find the source. Start off with everything clean. A slight ooze you won't find if sticky 5606 has already attracted alot of dust. Moving parts highest risk for a leak.

Check the swivel. There were just recent posts on how to repair this. Mine were part of the leaking with fair amount gummy in the region. Had them redone and pressure tested.

I thought I had it resolved with that. But no. Long story short, my calipers were leaking. I disbelieved them as a source because they had been "rebuilt" which involves replacement of the o-ring at the start of the problem. So before I got back to them I went through everything else due to softness and fluid loss. I rebuilt the parking brake valve (easy, just some o-rings), replaced multiple lines (don't forget there's a flexible line in the sidewall by rear passenger seats that won't last forever), redid my master cylinders and the lines from cylinders over to parking brake valve. With persistent softness and fluid loss I was stumped until one day I took the calipers off for the umpteenth time and noticed gummy around them. Viola, the leak. And it was only ever so small with oozing around the o-ring, cylinder bore and piston. The cause was corrosion "micropitting" of the cylinder bore. I have attached photo. I thought I could hone it out but the metal is soft and to get deep enough to get the pits out "wallowed-out" the caliper. I ended up having to buy new (it's own nightmare of availability, superceeded parts numbers, supply chain issues and limited production runs and waits).

Link to the micropitting picture:

http://www.337skymaster.com/messages...1&d=1649733211

When you fill the system, you have to back pressurize it to get all the air out. I used a garden sprayer with purpose built connector that connects to the bleeder from aircraft spruce. I also got their high end bleeder screws to put in the calipers all to reduce the chance of getting air in the system. When you rebuild the master cylinders, the gap on the piston/lock-o-seal is important.

[Dan schultz]

My parking brake valve was leaking on the inboard side, new seals did not fix it. I made a bushing and sleeved it, new seals good to go

Dan

[SierraYankee]

Greetings Folks;
anyone have a diagram or sketch of the C337 parking brake valve.?
Need to see if a worn valve could leak 5606 fluid simultaneously from both Left and Right hydraulic lines.
THANKS in advance

[Dan schultz]

Yes it can if the seals are bad. Mine had a bad bore and I sleeved it. Make sure to undo the fittings to get the small pins out so you can pull the actuator rod out. The actuator rod has the seals on it.

[SierraYankee]

Thanks for the info Dan.

[Kim Geyer]

I’ve had to reseal several valves. As a matter of fact I had a spare valve I would rotate around

[Charlie Lyford]

Hello,
I went through this on my P337. The parking brake valve can leak internally but not let fluid out. With one of the reservoirs being higher than the other the fluid gravity drains out of one and over fills the other. You will find the missing fluid in the insulation outboard of your left foot, it will hide a lot of fluid. New seals in the valve stopped the problem.