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I have a question concerning master cylinders and changing from manual brakes to power brakes. I understand, sort of, the difference between combination valves for disk/drum and disk/disk brakes. Where I am confused is if there is a difference between power drum/drum and disk/drum master cylinders. I have installed a booster and master cylinder from a 1972 F100 with disk/drum brakes, will this work? Or do I need to find a master cylinder from a power drum/drum setup? Any and all help and information will be greatly appreciated.
The drum-drum MC uses the same residual pressure for the front and rear ports. The disk-drum MC uses no residual pressure for the front, since disk brakes don't need it, and a residual pressure valve for the rear. Will it work? Yes, but the rear will lockup before the front engages enough.
I haven't had a disk/drum in my hand, but I believe you can see the residual valve inside the port. The disk/drum should have the insert on the small reservoir port and not have one on the large reservoir port.
Thanks for the info. I wonder if there is a way by part number to determine which use Master Cylinders and Combination/Proportioning valves are designed for. I am making modifications to a couple of trucks, 1972 & 1966, and trying to make sure I have a correct and safe braking system is getting frustrating. For some of the parts I obtain the original use cannot be determined.
Last edited by lauxg; Jan 6, 2016 at 08:36 AM.
Reason: missing word
A disc/drum MC will have a larger primary reservoir than the secondary reservoir. A drum/drum MC will generally have two equal sized reservoirs.
In the OEM design of the factory installed MCs, the ports of the MC feeding a drum brake circuit would have had Residual Pressure Check Valves or, an RPV (Residual Pressure Valve) just under the port seat(s) of the MC.
Today, if you buy a replacement MC from the parts stores, it's very probable the MC will not have any RPVs installed, even if the original MC for the application did. --cup expanders in the wheel cylinders have pretty much made RPVs, installed in the MC, obsolete.
Exploded view of a manual drum/drum MC.
Cup expanders press against the lip of the cups to keep them tightly sealed to the wheel cylinder bore.
Cup expanders can be metal discs between the ends of the coil spring and the cup seals or, the spring itself can have tightly wound conical shaped ends that press the cup seals into the cylinder bore walls.
Ultra Ranger, been following this thread and have a question. I would like to go duel system on my 66 F-250 and keep it drum/drum. If I convert and not knowing whats inside my wheel cylinders, I suppose I should remove them and inspect for cup ex panders. Do you agree with that? If I remember correctly, rebuild kits come with ex panders and brand new cylinders would only have cups and pistons
Ultra Ranger, been following this thread and have a question. I would like to go duel system on my 66 F-250 and keep it drum/drum. If I convert and not knowing whats inside my wheel cylinders, I suppose I should remove them and inspect for cup ex panders. Do you agree with that? If I remember correctly, rebuild kits come with ex panders and brand new cylinders would only have cups and pistons
It would not hurt to see what components are inside the wheel cylinders. It will also give you the opportunity to visually check for pits inside the cylinder bores.
Originally, RPVs had two purposes. Prior to the 1960s, MCs were mounted under the floor board. This placed the wheel cylinders at or, in some cases, below the level of the MC. Without RPVs, the brake fluid could siphon out of the wheel cylinders and back into the MC. This would cause the pedal to have to be pumped to refill the cylinders.
After auto manufacturers started mounting the MCs high up on the firewall, this eliminated the problem of drain back. The only purpose for the RPV after that was to keep the cup seals tight to the wheel cylinder bore, to keep air from being drawn in around the cups and into the wheel cylinder bores.
The advent of cup expanders has pretty much done away with RPVs (duck-billed, spring-loaded check valves inside the MC ports). RPVs are more susceptible to clogging up with rust scale and debris from the MC, are harder to service and more expensive to replace than cup expanders.
It would not hurt to see what components are inside the wheel cylinders. It will also give you the opportunity to visually check for pits inside the cylinder bores.
Originally, RPVs had two purposes. Prior to the 1960s, MCs were mounted under the floor board. This placed the wheel cylinders at or, in some cases, below the level of the MC. Without RPVs, the brake fluid could siphon out of the wheel cylinders and back into the MC. This would cause the pedal to have to be pumped to refill the cylinders.
After auto manufacturers started mounting the MCs high up on the firewall, this eliminated the problem of drain back. The only purpose for the RPV after that was to keep the cup seals tight to the wheel cylinder bore, to keep air from being drawn in around the cups and into the wheel cylinder bores.
The advent of cup expanders has pretty much done away with RPVs (duck-billed, spring-loaded check valves inside the MC ports). RPVs are more susceptible to clogging up with rust scale and debris from the MC, are harder to service and more expensive to replace than cup expanders.
Thanks for this reply, it offered much I never knew or would have thought of
Thanks for this reply, it offered much I never knew or would have thought of
I didn't state that quite correctly. What I meant to say, in the case of the MC being mounted under the floor board, is it put the MC at, or worse, BELOW the level of the WHEEL CYLINDERS. This would cause problems of drain back from the wheel cylinders to the MC.
wow here are some pics.... I will read all the reply's and try to answer.... thanks for all the posts answering FuzzFace no we never adjusted the rear brakes first...correct they are cylinders not calipers.... and NO bench bleed....2nd and third pic show that adjustment pot.....
The people at Pirate Jack seem pretty helpful and they offer some phone tech support I suppose I should call and see how they respond to the questions I posed here...... to those who inquired about the drum/drum disc/drum options etc.... I did have to confirm I got a kit that would work as someday I would want front discs maybe, so that MC will do front discs or front drums...
There are a couple of shops here that have old Chevy and Ford trucks parked there so I will most likely take it in to pressure bleed and have them adjust them....
The only OTHER ISSUE is it seems that the pedal is really high up away from the floor board...
I think the Pirate Push rod may have been longer...
Well the guy at Pirate Jack brought up a very interesting topic that I was clueless to. The PEDAL RATIO.... he said when going from manual to 'power brakes' there very well could be a difference in the pedal ratio... he mentioned 4:1... so googling master power brakes calculating pedal ratio brought up quite a bit of info.
He seemed to think since initially when I apply the brakes I get a good bight, it may not be the bleeding,,,,
I think you are replying to the wrong thread from 2016 and instead want to be using your new thread at this link. I was trying to figure out the context of your messages and talking about FuzzFace so I assume this is the one you wanted.
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