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Old 04-18-2011, 07:26 AM
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4 wheel manual disc questions.

For my 79 Malibu, running stock discs on front and gm metric discs in rear. I've ordered a stock manual brake master cylinder. From what I've been reading I need to pull the residual valve out of the master cylinder correct? Now, do I need to modify the proportioning valve and if so how? Trying to do this on a budget. Should I remove the residual valve and try it as is and modify the prop valve if necessary?

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Old 04-18-2011, 12:01 PM
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Why not just use a master from a vette, these are 4 whl disc brakes use its proportioning valve?
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Old 04-18-2011, 12:50 PM
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Do you know what year corvette? I don't want one made of cast iron.
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Old 04-18-2011, 03:33 PM
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Then use an S truck master cyl. The later ones (about 2002 up) have 4 whl disc brakes, easy to find at your local pick a part. Again get the proportioning valve too.
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Old 04-19-2011, 06:52 AM
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Thanks I'll look into it. All the applications I could find called for power brakes though, I'll look again.
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Old 04-19-2011, 02:47 PM
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The manual master cyls are the same as power master cyls. If you don't want the booster then don't use it. Whats wrong with power brakes anyways, if it were me I would use the works of an S truck or astro van.
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Old 04-21-2011, 08:38 AM
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Quote:
Originally Posted by Two8tyThree
The manual master cyls are the same as power master cyls. If you don't want the booster then don't use it. .
Not true, the piston size is different and also there are differences in the the depth of the actuation rod.

Vince
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Old 04-21-2011, 04:36 PM
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On an S truck this is true, the masters are the same.
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Old 07-02-2012, 09:25 AM
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I ended up using a Dodge Daytona Z master cylinder with 2lbs residual valves and no proportioning valve. I also installed a summit adjustable prop valve for the rear brakes but haven't put enough road time on it to dial it in for now though they work very well and give good pedal feel.
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Old 07-02-2012, 10:36 AM
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Quote:
Originally Posted by 6426yy View Post
I ended up using a Dodge Daytona Z master cylinder with 2lbs residual valves and no proportioning valve. I also installed a summit adjustable prop valve for the rear brakes but haven't put enough road time on it to dial it in for now though they work very well and give good pedal feel.
Unless the master cylinder is lower than the calipers, you should not need a residual pressure valve.
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Old 07-03-2012, 07:47 PM
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Cobalt,
I had really soft pedal pressure before and I even called Quick Performance and they told me that some guys use the Res valves so I tried it and it cured my problems. I'm just glad there finally working. I thought I'd share incase anyone else wanted a cheap alternative to the wilwood master.
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Old 07-04-2012, 05:43 AM
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A few thoughts:

Quote:
Originally Posted by 6426yy View Post
Cobalt,
I had really soft pedal pressure before and I even called Quick Performance and they told me that some guys use the Res valves so I tried it and it cured my problems. I'm just glad there finally working. I thought I'd share incase anyone else wanted a cheap alternative to the wilwood master.
Don't take this as an argument, OK? Consider it food for thought. You might know all of this but maybe it'll help someone doing a search.

On a normally functioning disc brake system having the MC above the calipers, there is no need for a RP valve. That's just the facts as I know them, and is backed up any number of times by brake manufacturers/distributors. If there actually IS a need, I would be looking for the root cause, rather than putting a 2 psi band aid on it.

The reason a RP valve shouldn't needed is because disc brake calipers have no retraction mechanism like a drum brake. A drum brake has springs that pull the shoes away from the drum, calipers don't. It would take 'vacuum' applied to the brake fluid to cause the caliper piston to retract- and that's what happens when the MC is BELOW the calipers and the fluid tries to run "downhill" to the MC.

The problem sometimes encountered when using a disc/drum MC on a 4-wheel disc system is inadequate volume to the rear brakes. Calipers take more volume than drums, if there's not enough volume, the pedal will bottom before the brakes are fully applied and the pedal will not be hard. The MC needs to be capable of about 1200 psi.

Providing all that is covered, to using the MC you bought you need to remove the residual pressure valve to the rear drums. The MC bore needs to be about 7/8" to 1" and the pedal ratio needs to be around 5:1.

If you have a soft pedal w/a 4-wheel disc non assisted brake system it could be due to air in the lines- bleeder screws on the bottom (calipers swapped side to side) will cause air to be trapped and a soft or spongy pedal. A too-high pedal ratio, a defective MC, or too small MC bore will also cause it.
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Old 07-04-2012, 11:27 AM
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Thanks Cobalt,
I'm not into arguing, And I agree with everything your saying. I've been reading up on this since I did the swap. The master I'm using is a 23 or 24mm bore can't remember and I switched to the manual pivot hole on the brake pedal arm. I think it is 1:6 but that's reading someone elses figure not mine. I know all the bleeders are up and I bleed and bleed these to no avail. I used a mityvac like 3 times and then had my stepson help me do it the old fashioned way and there were still soft and wouldn't get harder if you pumped them up. So the res valves I'm using won't actually hurt anything will they? This is mainly a toy and I hope to run it 1/8mi a lil bit.
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Old 07-04-2012, 04:54 PM
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Quote:
Originally Posted by 6426yy View Post
Thanks Cobalt,
I'm not into arguing, And I agree with everything your saying. I've been reading up on this since I did the swap. The master I'm using is a 23 or 24mm bore can't remember and I switched to the manual pivot hole on the brake pedal arm. I think it is 1:6 but that's reading someone elses figure not mine. I know all the bleeders are up and I bleed and bleed these to no avail. I used a mityvac like 3 times and then had my stepson help me do it the old fashioned way and there were still soft and wouldn't get harder if you pumped them up. So the res valves I'm using won't actually hurt anything will they? This is mainly a toy and I hope to run it 1/8mi a lil bit.
I believe you have bled them right, if they weren't you'd still have a soft pedal, even with the RP valves.

My worry is that there will be some drag on the rotor from the calipers having a RP valve, and that drag equals heat. That said, if the way you're configured allows there to be a RP valve and NO friction from the pads dragging on the rotors, you can assume there shouldn't be a problem. I'm just the type who would be wondering the WHY part of the equation- as in why does the RP valve help; what is the root cause that the RP valve is "correcting".

In any event, I would recommend you keep a close eye on the rotor/caliper temps as you go forward and start putting miles on it. Even a slight drag can, over time, overheat the brakes and that can lead to fluid boiling (mushy brakes all over again), warped rotors, and even cooking the wheel bearing grease.

You should be able to keep tabs on this by just feeling the outside of the wheel and caliper. Don't touch the rotor right after a drive, even if things are normal they'll be hot after a stop. It will be normal for the front brakes to be hotter than the rears, being as how the front brakes are about 70% of the stopping power of the brake system. If you think the rears are as hot or hotter than the fronts, that- to me- would indicate a possible problem that needed immediate attention. This would be an ideal place for an infrared thermometer. Harbor Freight has the Cen-Tech p/n 93984 for $20
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