Self applying brakes

bob ward
bob ward Senior Contributor
edited November -1 in HUDSON
Last weekend I had an unusual occurence in my 53, the brakes came on all by themselves.

I was on the highway, cruising 65-70mph, probably had not used the brakes for an hour, when I smelt hot brakes. Pulled over to investigate and sure enough the brakes were dragging and the brake lights were on, indicating that the system was pressurised. The handbrake was definitely off.

I scratched my head, jacked up the side of the car to crawl under and check that the brake linkage adjustments had not come loose and moved (they had not)

After maybe 15 minutes the brake lights went out and the brakes were no longer dragging. Checked the brake fluid in the master cylinder and it was still full. Scratched my head again and drove home another 4 hours with no more problems.

My theory is that the problem was caused by a combination of 2 things, high ambient temperature and lack of venting in the master cylinder cap. It was a fairly warm day, no cloud cover, 90F in the shade, so with engine heat and road heat, the master cylinder would have become quite hot, in turn heating and pressurising the air above the fluid in the master cylinder, which in turn pressurises the system and applies the brakes.

In general, different brake systems seem to have a tiny vent in the master cylinder top to allow small air movements but not the one on this car, or the 2 or 3 other Hudson master cylinders I've looked at since.

This car has a new brake sytem, maybe 2000 miles old, SS sleeved wheel and master cylinders with new kits, new flex hoses, old fluid flushed out.

Has anyone else had this experience or have alternative theories?

Comments

  • SuperDave
    SuperDave Senior Contributor
    Yep,

    Had the same thing happen on my 49 super six. Same rebuilt stuff stainless etc. The first indication I got was after a few stops, the car didn't seem to coast very easily.. brakes dragging a little. By the time I would jack it up all was well???? Then it got really bad one day. I found that the master cylinder had been assembled incorrctly and the piston was not returning to it's complete "off" position. (Improper linkage adjustment could do this, but not in my case) My problem came up after driving around town one day..Hot weather, (Florida in June) lots of stops and some fast driving to boot! My master cylinder cap was gone! Must have blown off or fell off. There was a small rubber washer installed in the wrong place, keeping the piston from returnming all the way. Sorry I can't recall the details. I reassembled the MC, bled all the fluid out until it was clear. I had no idea how long the cap had been missing and for sure the fluid had absorbed much moisture.. That probably added to the problem. Since then, I have driven several thousand miles with no dragging brakes. Now I still need a cap for the MC. I borrowed one from the 54 I am working on. I bought two since then. One from Kostancik and one from Albright. Both are too large diameter. That's what I get for thinking they are all alike! Anybody have a small one to trade for two large ones? LOL..
  • dave s
    dave s Senior Contributor, Moderator
    You didn't say what kind of brake fluid you used. If silicon, it probably expanded and caused your problem. Wnen it cooled, they released. We had same problem with my son's 54 Hollywood with power brakes. Whenever he took car on trip, rear brakes would lock up. If we opened master cylinder cap, they released. Flush all silicon out of system and replace with DOT 4. I have been using DOT 4 for years with no problem.
  • rambos_ride
    rambos_ride Senior Contributor
    SuperDave wrote:
    Yep,

    Had the same thing happen on my 49 super six. Same rebuilt stuff stainless etc. The first indication I got was after a few stops, the car didn't seem to coast very easily.. brakes dragging a little. By the time I would jack it up all was well???? Then it got really bad one day. I found that the master cylinder had been assembled incorrctly and the piston was not returning to it's complete "off" position. (Improper linkage adjustment could do this, but not in my case) My problem came up after driving around town one day..Hot weather, (Florida in June) lots of stops and some fast driving to boot! My master cylinder cap was gone! Must have blown off or fell off. There was a small rubber washer installed in the wrong place, keeping the piston from returnming all the way. Sorry I can't recall the details. I reassembled the MC, bled all the fluid out until it was clear. I had no idea how long the cap had been missing and for sure the fluid had absorbed much moisture.. That probably added to the problem. Since then, I have driven several thousand miles with no dragging brakes. Now I still need a cap for the MC. I borrowed one from the 54 I am working on. I bought two since then. One from Kostancik and one from Albright. Both are too large diameter. That's what I get for thinking they are all alike! Anybody have a small one to trade for two large ones? LOL..



    Can you give me a measurement of the diameter of the cap? I have a stock MC from my 49 that is getting replaced and if its the right one its yours!
  • SuperDave
    SuperDave Senior Contributor
    rambos_ride wrote:
    Can you give me a measurement of the diameter of the cap? I have a stock MC from my 49 that is getting replaced and if its the right one its yours!

    Rambo, I'll take you up on that one! Glad to make a contribution to the 49 restoration.. BUT I will measure THIS time and let you know. The cars are in an industrial building a few miles away. I usually go down there to do some fixoration on the 54 Hollywood at least once per day.

    Thanks, Dave W.
  • Unknown
    Unknown
    i have had this problem several times after installing a new master cyl, the solution is to shorten the rod that pushes into rear of cyl, the new ones are usually set too long preventing recirculation of brake fluid in master cyl, I now always use the original rod and throwing away the new one that comes with master cyl. THIS SHOULD SOLVE YOUR PROBLEM. FIRST INDICATION IS BRAKE LIGHTS STAY ON AFTER LIGHT USAGE, SHOWING A PRESSURE BUILD UP GOOD LUCK, BILL ALBRIGHT
  • nick s
    nick s Senior Contributor
    dave s wrote:
    You didn't say what kind of brake fluid you used. If silicon, it probably expanded and caused your problem. When it cooled, they released. We had same problem with my son's 54 Hollywood with power brakes. Whenever he took car on trip, rear brakes would lock up. If we opened master cylinder cap, they released. Flush all silicon out of system and replace with DOT 4. I have been using DOT 4 for years with no problem.

    let me elaborate on the experiences with my hollywood. as i have seen both assembly and silicon problems.



    the total lock-up at the master cylinder was a one time occurence. loosening the cap allowed the master to vent and the brakes released. subsequent problems were finally traced to a cocked vacuum booster piston. which could allow the breaks to be slightly energized.

    the master must be able to return to its fully retracted position to allow the lines and reservoir to equalize pressures. on my father's 54 power brakes, this would not happen due the addition of a hose clamp on the internal vacuum line prior to his purchase.



    with the lesser expansion of dot 3 fluids, it may not have been necessary to vent to atmosphere. silicon fluid did play havoc on stoplight switches. since switching to dot 4 about 10 yrs ago, i have not had to replace my switch after 1-2 a year with silicon.
  • bob ward
    bob ward Senior Contributor
    Thanks for all the responses. I've increased the amount of play in the brake linkage to eliminate the possibility of a linkage problem. I can with little trouble wire a makeshift brake light into the cabin to give me early warning of self pressurisation. The brake fluid used was a standard fluid, not a silicon fluid.
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