| Switching Brake Lines on Master Cylinder [message #270902] |
Sat, 31 January 2015 19:58  |
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John Sharpe
Messages: 71
Registered: February 2006
Location: Porter, TX
Karma: 0
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I installed a so called "P30" master cylinder on my coach. In order for the lines to hook-up I had to switch the front brake line to the secondary bore and the rear brakes to the primary piston in the master cylinder. This means the pressure in the rear brakes applies the front brakes. Did GM have a reason for engineering it so the pressure in the front brakes drives the piston for the rear brakes? I know the combination valve holds off delivery of pressure to the front brakes until the pressure is 300-400 psi. Any Hydraulic engineers out there?
John Sharpe
Porter, TX
78 Eleganza II, TBI
40 Ford Panel, TPI
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| Re: Switching Brake Lines on Master Cylinder [message #270912 is a reply to message #270906] |
Sat, 31 January 2015 22:34  |
Larry
Messages: 2875
Registered: January 2004
Location: Menomonie, WI
Karma: 10
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John Biwersi wrote on Sat, 31 January 2015 21:26John. I "think" GM's reasoning was related to reserve brake fluid volume requirements. The front calipers require more reserve brake fluid than the 4 rear cylinders and the master cylinder they used that fit under the hood has a larger reserve at the rear chamber.Thus the rear chamber supplied the front calipers. The P30 (34mm) master cylinder has the same brake fluid reserve capacity in both chambers so again I "think" you can connect the brake lines either way. Were you able to bench bleed the master? Some members have had lots of trouble getting it to bench bleed. Be very careful that no junk gets into either chamber that could prevent the tipping inlet valves from closing. If they don't close you could have nothing for brakes. Contact Bob Miller about loosing brake capacity with the 34mm master.
I'm with John on this. Bench bleeding is very important with these "P30's". I went through 5...yes count them...5 P30 MC's. On bench bleeding, I could not get the front piston to bleed. Good flow through the rear. After talking with Dave L, he had the same issue. He then levered the plunger and was able to put enough pressure on it to break the front piston loose...then it bled properly. Conclusion we came to was that the "P30" is not an item that is a "mover" anymore, so the ones I got had been sitting on the shelf for extended periods and the front piston and cups had frozen to the bore. I ended up putting in a stock used MC that I borrowed from a friend, and all of my brake issues went away. IMO, while you may have a slightly lower pedal, the stock MC will give you the most line pressure because of the smaller diameter piston. JMHO
Larry 
78 Royale w/500 Caddy
Menomonie, WI.
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