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BN1/2 Brake Residual Pressure

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Big End

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I have a 1955 Healey BN1, which has retrofitted BN2 front drum brakes. I have a problem with dragging front brakes, rears seem fine. I can pump the brake pedal as much as I like with the car stationary, except after a run. The brake pedal then has less free movement, the brake lights remain on, and the front brakes become very hot, a sign of residual pressure in the system. Initially I thought this might be due to a blocked return port in the (new) master cylinder, but this seems unlikely as it would affect all 4 wheels. I’m zeroing in on faulty front brake pipes, which were recently replaced, but which might be restricting return fluid when the brakes are hot or have been used for hard braking. Light braking causes no adverse symptoms, but hard braking seems to cause residual pressure in the system. Does anyone have any ideas about what might be going on?
 
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This fault is usually attributed to internally collapsed flex lines, but usually only affects one wheel. Did you replace both flex lines? Did the problem start after you replaced the front 'pipes?' If so, I'd suspect a kink in the hard line that supplies both front wheels.
 
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Big End said:
I have a 1955 Healey BN1, which has retrofitted BN2 front drum brakes. I have a problem with dragging front brakes, rears seem fine. I can pump the brake pedal as much as I like with the car stationary, except after a run. The brake pedal then has less free movement, the brake lights remain on, and the front brakes become very hot, a sign of residual pressure in the system. Initially I thought this might be due to a blocked return port in the (new) master cylinder, but this seems unlikely as it would affect all 4 wheels. I’m zeroing in on faulty front brake pipes, which were recently replaced, but which might be restricting return fluid when the brakes are hot or have been used for hard braking. Light braking causes no adverse symptoms, but hard braking seems to cause residual pressure in the system. Does anyone have any ideas about what might be going on?
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What brake fluid are you using? DOT5 causes a problem because of the design of the BN1/BN2 master cylinder.
 
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John Turney said:
What brake fluid are you using? DOT5 causes a problem because of the design of the BN1/BN2 master cylinder.
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I’m using DOT 4 fluid, not had a problem with it up to now. What problems does the DOT 5 fluid cause? I have a new master cylinder on the car, but I doubt if that is the problem, as only the front brakes are binding. M/C problem would surely affect all of them, I guess.
 
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Bob_Spidell said:
This fault is usually attributed to internally collapsed flex lines, but usually only affects one wheel. Did you replace both flex lines? Did the problem start after you replaced the front 'pipes?' If so, I'd suspect a kink in the hard line that supplies both front wheels.
Click to expand...
I’m wondering if clamping the new flexible hoses while servicing the front brakes might have caused an internal partial blockage in one of the pipes. But, if that was the case, why does the residual pressure only show up when I’m on a run, and using “full” braking? I can press the pedal as hard as I like before a run, and the pressure remains normal. I judge this by seeing if the brake light pressure switch has activated. The brake lights are on all the time on a run when the pressure has built up and the brakes start binding.
 
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Big End said:
I’m using DOT 4 fluid, not had a problem with it up to now. What problems does the DOT 5 fluid cause? I have a new master cylinder on the car, but I doubt if that is the problem, as only the front brakes are binding. M/C problem would surely affect all of them, I guess.
Click to expand...
DOT 5 will cause swelling of the "rubber" seal in the master cylinder, which blocks the return hole. It only affects the BN1/2 MC.
 
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If the issue was caused by the swelling of the front rubber brake lines, then the brake light switch wouldn't be seeing the pressure. On the 3000, the pressure switch is on the union that supplies pressure to all brakes. Assume the 100 is the same. Sounds like the issue is with the master cylinder.
 
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That makes sense. I’m once again zeroing in on the master cylinder or actuating rod. I still can’t understand why the brakes are normal under light use, but drag once hard braking has been done, and they have got hot.
 
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