OK folks I'm thinking real hard now and you all know that's dangerous.
Imagine a simple closed hydraulic system with a large diameter plunger at one end (the slave at the disks) and a smaller one at the other (the master). Put the system under pressure by heat. Pressure and thus volume is distributed evenly throughout and so the smaller plunger will move further than the large one and the latter does not lock up on the wheel because once it touches any pressure and volume is dissipated at the other end. I suspect this is now the Great Plains system operates,ie., there are no relief ports. However, what if the smaller plunger is prevented or retarded by moving because it is simply attached to a relatively heavy articulated brake pedal or alternatively is overfilled against its stops. Pressure is then applied in relatively greater amount at the disk and lock up occurs. Very little pressure is needed as evidenced by the minute amount of fluid that escapes when I release the pressure at the slave bleeder. Now apply a relief port towards the end of the master that normally allows fluid to spill around the smaller plunger and return to the resevoir in the event of heat to overcome the above scenario. What if the plunger doesn't return sufficiently to uncover the port because of the pedal weight. Bingo, lock up again. Next point, the Great Plains system doesn't have a resevoir in the typical sense.If a relief port exists, then at rest and under heat, some fluid must spill around the plunger via the port to a second internal void somewhere else. There are no leaks so it must go somewhere! If this void is overfilled during bleeding then the spill can't occur again leading to lockup. What happens over a period of hot weather? A little bit of fluid gets pushed out the port into the void each time but because my cylinder is vertical, it might not get sucked back as the system cools (this would depend on the port location relative to fluid level in the void). Next time the brake is used the plunger moves in relatively further to take up the reduced volume of fluid and eventually the port doesn't get uncovered on release. Again potential lockup. My solution, yet to be tried, is to put a penny washer across the end of the master with a compression spring against it and around the push rod with the other end of the spring at the pedal fitting and to ensure enough play in the system by not overfilling. Hopefully, the spring will assist the master plunger in returning fully and uncovering the relief port. I'll let you know if it works. Comment welcome on all this conjecture. Anyone know of a fluid that doesn't expand at all under heat. We're having a real hot spell here. 35-40 C every day lately. John Martindale Family 29 Jane Circuit TOORMINA NSW 2452 AUSTRALIA ph: 61 2 66584767 email: [email protected] web: http://www.members.optusnet.com.au/johnjanet/Martindale.htm
