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dwcsr · 06-22-2006, 12:43 AM

Just some notes I researched while waiting on parts.

The rear brake line port on master cylinders are not the same through a maker or style, some will have different size thread and some have the same size threads. Some have rear port to the rear brakes and some have front port to the rear brakes. I must have searched 400 sites before I saw a pattern.

The easiest way to tell visually is the size of the bowls. Larger one goes to the front Disc and in the event of same size bowls the rear brake port will have a residual valve in it and a larger brake line if it’s a Disc/Drum system. The master cylinder ports on most late model cylinders have a built-in residual valve in the drum side. Early drum/drum have one in each port. It needs to be removed on a Disc/Disc system. Disc/Disc corvette master cylinder is rear port to rear brakes. It seems that falls true for all disc/disc systems but I can't say for sure.

On the Disc/drum system there needs to be a residual pressure on the wheel cylinder pistons so they don’t return completely as you let off the pedal and sucking the pistons back into the wheel cylinder bores away from the shoe push rods. This would make for a low or slow responding brake application because it has to push the wheel cylinders piston out to contact the push rods and then engage the drum and then build line pressure to engage the discs. The residual valve keeps the wheel cylinder pistons extended and in contact with the push rods to the point that 10 psi of back pressure is needed to retract them and they will not be sucked back in when letting off the brakes as the fluid moves backward into the line.

Disc/Disc does not have this problem because the resistance of the big o-ring against the caliper piston is greater than the vacuum created by letting off the pedal. The disc out of round actually pushes the piston back in. there is one exception with GM low drag calipers which retract further than normal. They were early 80's cars and possibly trucks.

In the first pic you see restrictor plate in the rear port; the residual valve would look much like this. In the port hole in pic 2 you see nothing.

06-22-2006, 12:43 AM	#17
dwcsr Hollister Road Co. Join Date: Apr 2005 Location: Houston Posts: 6,134	Re: Hydroboost on a 58 -59 3100 Just some notes I researched while waiting on parts. The rear brake line port on master cylinders are not the same through a maker or style, some will have different size thread and some have the same size threads. Some have rear port to the rear brakes and some have front port to the rear brakes. I must have searched 400 sites before I saw a pattern. The easiest way to tell visually is the size of the bowls. Larger one goes to the front Disc and in the event of same size bowls the rear brake port will have a residual valve in it and a larger brake line if it’s a Disc/Drum system. The master cylinder ports on most late model cylinders have a built-in residual valve in the drum side. Early drum/drum have one in each port. It needs to be removed on a Disc/Disc system. Disc/Disc corvette master cylinder is rear port to rear brakes. It seems that falls true for all disc/disc systems but I can't say for sure. On the Disc/drum system there needs to be a residual pressure on the wheel cylinder pistons so they don’t return completely as you let off the pedal and sucking the pistons back into the wheel cylinder bores away from the shoe push rods. This would make for a low or slow responding brake application because it has to push the wheel cylinders piston out to contact the push rods and then engage the drum and then build line pressure to engage the discs. The residual valve keeps the wheel cylinder pistons extended and in contact with the push rods to the point that 10 psi of back pressure is needed to retract them and they will not be sucked back in when letting off the brakes as the fluid moves backward into the line. Disc/Disc does not have this problem because the resistance of the big o-ring against the caliper piston is greater than the vacuum created by letting off the pedal. The disc out of round actually pushes the piston back in. there is one exception with GM low drag calipers which retract further than normal. They were early 80's cars and possibly trucks. In the first pic you see restrictor plate in the rear port; the residual valve would look much like this. In the port hole in pic 2 you see nothing. Attached Images Last edited by dwcsr; 07-15-2006 at 09:55 PM.