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Thread: Brake fluid is leaking from parking brake valve PA18

  1. #1
    cubnut93's Avatar
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    Brake fluid is leaking from parking brake valve PA18

    A friend of mine has a PA18 that is leaking brake fluid from the parking brake valve. Does anyone have some drawings of this valve? Mainly interested in the internal parts and how there are put together.

    I have ordered the two O-rings that are inside of the valve. I am hoping that replacing them will take care of the problem.

    Has anyone out there replaced the O-rings in this valve?
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    You could give me a diagram of a brick and I’d still have trouble reading it.
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    Crash, Jr.'s Avatar
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    I rebuilt mine a month or so ago. I have a bunch of pictures and have been meaning to do a write up on how to rebuild them.

    That little o-ring is an inside o-ring in a very small bore and extremely hard to remove and replace. A good dental pick will get it out. Installing the new one involves folding the o-ring in half and inserting it then using the plunger and a poker, one on each side of the o-ring to get it placed over the grove and then gently unfolding it so it falls into the groove. I used the plunger shaft liberally greased with tach lube inserted backwards to finish seating the o-ring in it's groove in the bore.

    Easier said than done though. It's 30 minutes of frustration or the 2 beer equivalent. A good light, magnifying glass, and a set of o-ring picks are crucial. O-rings are MS28775-006, MS28775-010, and the crush washer for the main "plug" is AN900-10
    Last edited by Crash, Jr.; 06-08-2021 at 03:16 PM.
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    Univair gave me MS-28775-010 and MS-28775-113 for the O-rings. The AN numbers were AN6227-11 and AN-6227-5 according to them! I'll have to call them tomorrow and check those numbers again.

    Thanks for the reply.

  5. #5
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    [QUOTE=Crash, Jr.;804999]I rebuilt mine a month or so ago. I have a bunch of pictures and have been meaning to do a write up on how to rebuild them.

    ++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++

    Can the valve be removed from the master in the airplane or do I need to removed the whole assembly and remove the valve on the bench?
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  6. #6
    Crash, Jr.'s Avatar
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    The valve should ideally be removed from the plane and rebuilt on the bench. It would be extremely difficult to access it well enough to fish out little seals with it in the plane.

    It's fortunately fairly easy to just pull the diaphragm cover plate with the valve or the booster cylinder if you have boosters. You just need to be double jointed, willing to fold yourself up in the floor in front of the seat, and have a long screwdriver.

  7. #7
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    Quote Originally Posted by Crash, Jr. View Post
    The valve should ideally be removed from the plane and rebuilt on the bench. It would be extremely difficult to access it well enough to fish out little seals with it in the plane.

    It's fortunately fairly easy to just pull the diaphragm cover plate with the valve or the booster cylinder if you have boosters. You just need to be double jointed, willing to fold yourself up in the floor in front of the seat, and have a long screwdriver.

    Is it easier to remove the whole assembly from the plane and take the valve off the master on the bench?

    I know that there is a crush washer that seals the valve to the nut, but how is the threaded piece that goes between the master & the valve get sealed?

    The one thing that worries me about removing the entire assembly from the plane, is removing the rod that goes from the master pedal to the forward brake pedal. I don't want to bend that rod to be able to remove the entire assembly from the plane. I'll have to check and see if I can just remove the pivot pin/bolt so I don't need to bend that rod. I have had them break before.
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    The connection between parking brake and master cylinder is a 1/8" NPT close nipple.

    You (or your friend) might want to think about upgrading the brakes while you've got everything apart. I like Steve's vented brake boosters: https://www.stevesaircraft.com/vbrake.php

    And if you're worried about breaking the rods, you might also want to look at Dakota Cub's adjustable brake rods. https://dakotacub.com/index.php?rout...product_id=200

    Sometimes it's fun to spend other people's money.

    Is it easier to remove the whole assembly from the plane and take the valve off the master on the bench?
    Yes, definitely. Can't imagine doing it any other way.

    Oh yea, and think about replacing the flathead screws that hold the cover to the frame with allen head screws so you can get on them with a ball head. Plenty of frustrating things about this job.
    Last edited by fancypants; 06-10-2021 at 10:56 PM.

  9. #9
    Crash, Jr.'s Avatar
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    Brake boosters make a whole world of difference. I have the North River ones and they work fantastic as long as there are no leaks anywhere in the system. The reservoir is very small so any fluid level change can mess up the whole deal.

    Do be aware though, if you go to boosters there is likely to be some messing with fittings and brake line length due to the boosters being longer than the original diaphragm setup so the fittings have to move forward.

  10. #10
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    I have removed the master & parking brake assembly from the plane.

    See pix for more info & questions.

    The O-ring that is inside the valve body looks like it will be very hard to remove & replace--wish me luck!

    Univair DID send me one O-ring that was not correct--I have reordered the correct one. Thanks for posting the correct O-ring numbers.
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    I used a pick with a 90° at the end. I heated it up with a torch and melted the rock hard O-ring in several places, then I was able to blow it out with compressed air. worked great. Installing the new O-ring wasn't too bad. I used some of the suggestions in this post--thanks for them!

    Now I am waiting for the correct O-ring from Univair.

    BTW--I was some what surprised that I was able to get help with the O-ring numbers through Google. I Googled the AN2667-1 number and it came up with the new number, along with the dimensions at Spencer Aircraft Supply.
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  12. #12
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    Everything is back together with no leaks & proper braking!!!

    Thanks for every ones help!!

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    I recommend chemical extraction for the O ring inside the body. Once the unit is disassembled , submerge in MEK overnight , Use a wooden or plastic tool to extract the swelled softened oring. The interior cylinder walls of the valve must not be damaged.

  14. #14
    Crash, Jr.'s Avatar
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    If you're gentle and use a brass pick you can extract those o-rings fairly quickly without doing damage. A brass dental pick is a pretty hard to find but invaluable for work on delicate items.

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    While we are on the parking brake discussion. I just purchased a Super cub and it does not have the Scott brake valves on it. What is everyone’s opinion on them? Trying to see if it makes sense for me. Also to add to the discussion I have a stock Pa-18-95 no starter. I do see the benefit for me as an added safety feature for hand propping.

  16. #16
    Crash, Jr.'s Avatar
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    I added them to my J3 and they are by far the most used mod I've put on so far. It's nice to be able to just lock the brakes and hand prop.

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