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Old 09-24-2006, 10:11 PM
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disc brake conversion master cylinder

Hope you can help, I changed from a single chambered master cylinder to a dual and also installed disc brake setup using single piston GM calipers,had a hard time bleeding them but did get the pedal pretty firm but it seems the caliper pistons won't retract quite enough, they drag pretty bad. It's a non power assist (manual) master just wondering wrong one maybe, its a Corvette cylinder and seems to work but maybe it's not allowing enough fluid to drain back to the master? Just one of my thoughts,any help gladly welcomed. Eric

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Old 09-24-2006, 11:48 PM
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Quote:
Originally Posted by Eric64

Hope you can help, I changed from a single chambered master cylinder to a dual and also installed disc brake setup using single piston GM calipers,had a hard time bleeding them but did get the pedal pretty firm but it seems the caliper pistons won't retract quite enough, they drag pretty bad. It's a non power assist (manual) master just wondering wrong one maybe, its a Corvette cylinder and seems to work but maybe it's not allowing enough fluid to drain back to the master? Just one of my thoughts,any help gladly welcomed. Eric
Did you use the correct replacement M/C (Disc/Drum-Manual/Power)? Is the larger reservoir connected to the front disc? What type of valving (combination or separate metering/proportioning)? New brake hoses on the front?

How is the bleeding screw position on the front calipers? Do they point straight up to allow full discharge of air (you may have to loosen calipers and point bleeding screw straight up while bleeding).
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Old 09-25-2006, 12:45 AM
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Check the length of the pushrod from the peddle to the master cylinder piston. It might be too long.
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Old 09-25-2006, 02:14 PM
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Quote:
Originally Posted by Triaged

Check the length of the pushrod from the peddle to the master cylinder piston. It might be too long.
The most obvious and needed to be checked first defect...
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Old 09-25-2006, 04:08 PM
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Assuming the brakes are bled adequately, do you have residual pressure valves in the system? You don't mention what it is you did this to, but it sounds as though you have converted from drums to disc. If there were residual pressure valves in the drum system, they are most likely the wrong ones and would not cause enough pressure to be released in the disc system. Drums use a 10 lb valve, while discs use a 2 lb. Just a thought and worth checking.
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Old 09-25-2006, 08:46 PM
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As far as I know, residual valves are used only when the M/C is at or below the level of the drums or calipers. Most cars that have the M/C under the floorboard use residual valves.

Vince
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Old 09-25-2006, 09:29 PM
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You didn't mention whether you have installed a proportioning valve or not ... but I'll assume that you have, or should have ... with disc front, drum rear.

This How Stuff Works Article confirms (I hope) my understanding of what I think your problem might be.

If you do have a proportioning valve, have a look at it. There should be a little reset pin under a rubber boot at one end of the valve.
It's the black boot with the red tag on the RH side of this picture.

If someone has pushed too hard or too quickly on the brake pedal in an un-bled system they may have "tripped" the pressure differential valve ... cutting off the flow of fluid to either the front or the back fluid circuit.

Pushing the pin back in re-centers the valve and allows flow to both circuits.

HTH,
Don
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Old 09-26-2006, 02:44 AM
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Quote:
Originally Posted by 302/Z28

As far as I know, residual valves are used only when the M/C is at or below the level of the drums or calipers. Most cars that have the M/C under the floorboard use residual valves.

Vince
The font disc 2 lb Residual Valve (disc) would be used on disc only if the M/C is installed below caliper level (to prevent gravity flow fluid drainback to M/C). The 10 lb residual valve is used on all rear drum (disc/drum) to keep the shoes slighly spread to lessen the lead time to have them apply before the front disc makes contact (further retarded by metering valve).

The 10 lb. residual valve is usually incorporated in either the M/C or proportioning valve (or possibly both when using universal parts fit).

As mentioned, the pushrod adjustment should be the first thing to check (IMO)
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Old 09-26-2006, 12:46 PM
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If you have below the floorboard MC, you need the residual pressure valves as others have pointed out.

You do NOT need any in the MC as well and if it is there should be removed.

If the MC RPV's remain in place, you will have delayed pedal and therefore slow MC/Caliper piston return allowing the brakes to drag -

Dave
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Old 09-26-2006, 03:19 PM
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Thanks for all the info, I haven't got back into it as of yet there are no external residual valves or prop. valves. I thought I would first get it so I can move it to another location to really work out the problems and this was the first I found. I'm waiting for a new master clyinder to arrive just to eliminate that as the problem and I'm going to pick up a residual valve for the rear since it's not operable as of yet. Thanks again for the thoughts.
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