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Thread: master cylinder

  1. #1

    master cylinder

    Ron, the left brake on the pilot side is connected to the left brake on the co-pilot side by a shuttle valve, but not to the right brake on either side. My guess is that you didn't get a good rebuild on your master cylinder or didn't get all the air out. A score in the cylinder, a cut o-ring or a bad o-ring might cause the problem. Some leaking out the top is expected if you fill the cylinder too full (hard not to do!). This assumes that your Sundowner is the same as my Musketeer.

    ----- Original Message ----
    From: ron95020 <>
    Sent: Thursday, November 9, 2006 9:11:35 PM
    Subject: [musketeermail] master cylinder

    I have a 73 Sundowner with master cylinders on both sides. I just
    rebuilt the ones on the left side. They were leaking fluid. I noticed
    it was a bit harder than normal to bleed the fluid. I am having
    trouble with the left one getting soft. I am wondering what the left
    side has to do with the right side. When I try the right side brakes
    it feels good, when I try the left side the left one is soft, but not
    all the time. Anyone that can help with this? Should I rebuild the
    right side master cylinder also?

    Thanks for any help in this matter.


  2. #2
    The Sundowners all have remote brake fluid reservoirs, as opposed to integral reservoirs on the master cylinders like the earliest Musketeers. Beech made the switch in mid-year 1967. Some 1967 planes sill have the early style, and some have the later style; all the 1968 and later planes had the later style.

    When bleeding the brakes, it needs to be done by pumping fluid into the bleeder valve on the wheel cylinders. It helps to have someone else use their hand to slowly 'jiggle' the rudder pedal a bit during the bleeding, to dislodge air bubbles in the cylinder. They cannot push fast or hard, or the person at the bleeding screw will get an Mil-H-5606 bath.

    If you have not repaired a master cylinder before, and did not have oversight while you did it, you may have missed the check valve function at the base of the piston (or mis-assembled it). There is a small O-ring there, which operates as part of the check valve function. If not assembled properly, or if the ring is bad, it will either keep the cylinder from refilling, will allow it to bypass fluid, or will force it to suck in air instead of replacement fluid (when the pedal is released).

    There were four different versions of remote-feed master cylinders used on these planes, so I can't know which ones you have. They all supersede, and I have found planes with one of each type in service. The Cleveland 10-55 will replace all of them (remote feed only), but none of them are cheap anymore.

    What follows is part of a post explaining how to keep air from entering the system during cold weather.
    Since there was no sign of a leak anywhere, almost no oil missing from the reservoir, and a quick bleeding fixed it, my bet is that you had air in the cylinder. The reservoir level slowly goes down as the pads wear; so it is normal to find it slightly low.

    When the master cylinders age, the o-rings harden. The problem most often arises in winter, or following a flight at a cold altitude. When you land, the brakes work fine; but when you suddenly take your foot off the brake, the hardened and slightly shrunken o-ring in a master cylinder lets air get sucked in (rather than keeping things sealed until fluid can get sucked in). This is a normal tendency that causes few problems on the older planes, until the cylinders begin to leak.

    But for some inexplicable reason, on the later planes, Beech moved the brake fluid reservoir very low on the firewall. If you stand back and look at the plane, and visualize where the master cylinder shafts pass through their seals, versus the height of the fluid level in the reservoir, you'll see that they are very nearly on the same horizontal plane. Since cold brake fluid tends to be sluggish, while air moves easily, the combination of suction (foot quickly removed from pedal) and the need to actually 'lift' fluid from the reservoir, lets air get past the o-rings.

    Installing new o-rings in the master cylinders is a temporary solution; and one that will eventually be needed anyway. But the permanent solution to this specific condition is to relocate the reservoir near the top of the firewall. All that it takes is installing a longer screw in place of one of the existing screws, in the desired clamp location. An all-metal locknut is used on the longer screw, basically turning the extension into a mounting stud. The reservoir is disconnected (after draining), and its clamp assembly is relocated to the new mounting stud. Then a new (longer) quarter-inch brake line is fabricated, to connect the bulkhead fitting to the relocated reservoir.

    This relocation will provide a constant 'positive head pressure' on the fluid replenishment lines feeding the master cylinders. In turn, this will virtually eliminate air being sucked in around the shaft seals.

    If my conjecture is correct, now that this has happened once, it will happen again. It will likely become more frequent as the weather cools. If you would like to bring the plane to 34A, and help with the work, I can help you change the master cylinder o-rings, and-or relocate the reservoir.

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