Which 7/8 master cylinder?

-
7

71Demon

Well-Known Member
Joined
Dec 23, 2005
Messages
550
Reaction score
40
Got 4 wheel manual wilwood disc brakes installed. Hard pedal and car just wont stop good. Thinking I have a 1-1/32 now. Hoping a 7/8 master cyl will help. Any suggestions on which one too use? I have seen some posts saying use a 7/8 for pwr brakes. Thanks
 
Explain what you have and your problem. He may be able to tell you what you need. Maybe even get it for ya.
 
I run 15/16” MC’s from DoctorDiff with my manual disks. I’ve used them with the standard 73+ Mopar single piston calipers on 11.75” rotors and DoctorDiff’s 13” cobra style kit with PBR calipers. Works great! I think it’s a really good compromise between pedal effort and the length of the pedal stroke. I’m sure 7/8” would work too, but the pedal stroke would be a little longer.
 
72bluNblu said:
I run 15/16” MC’s from DoctorDiff with my manual disks. I’ve used them with the standard 73+ Mopar single piston calipers on 11.75” rotors and DoctorDiff’s 13” cobra style kit with PBR calipers. Works great! I think it’s a really good compromise between pedal effort and the length of the pedal stroke. I’m sure 7/8” would work too, but the pedal stroke would be a little longer.
Click to expand...
 
Many here use the 15/16" Dr Diff booster w/ adapter plate (~$120) w/ manual disks.

I have a 7/8"D MC w/ on all 3 of my Mopars. I used one for a 1999 Breeze/Cirrus/Stratus w/ ABS. I am sure many others would work. Two have a 2-4 bolt adapter plate ($30 ebay), one manual and one on a factory booster. The other is on a 90's Intrepid booster (so no adapter needed) on Dart firewall brackets.
 
71Demon said:
Got 4 wheel manual wilwood disc brakes installed. Hard pedal and car just wont stop good. Thinking I have a 1-1/32 now. Hoping a 7/8 master cyl will help. Any suggestions on which one too use? I have seen some posts saying use a 7/8 for pwr brakes. Thanks
Click to expand...
With a 1&1/32 size bore, it may take more leg-power than you are used to.You might want a booster.
But if it doesn't stop "good" with a hard pedal, you might consider a problem in the install. I think I would get all 4 wheels up, and have a helper stand on the brakes while you do a walk-around testing each wheel for resistance. You may find one end or the other not working
 
I’m running Cass’s 15/16 manual master on the 70 Dart with Wilwood front disks and it works great.

Give him a call.
 
tjpatte said:
I’m running Cass’s 15/16 manual master on the 70 Dart with Wilwood front disks and it works great.

Give him a call.
Click to expand...

_1987 dodge diplomat, alum, 2 bolt, pedal feel is about twice what normal power brakes are. Used old splitter for the front / rear connection, no problems. Cut and thread original manual brake pushrod, put back together w/ a treaded rod coupling. adjust for adapter plate if u needed one to go from 4 bolt to 2 bolt . Weld , trim to look racy , before final install.
 
Thanks everyone for the replies. Any more info on Cass's contact? Dont know exactly whats going on but along with the car stopping slow the calipers right now are locked??? Any ideas?
 
71Demon said:
Thanks everyone for the replies. Any more info on Cass's contact? Dont know exactly whats going on but along with the car stopping slow the calipers right now are locked??? Any ideas?
Click to expand...
Locked brakes and non-returning pedal is fluid not returning into the reservoir, usually a bad hose..... But
did you pull the residual valve out of the port that goes to the front brakes?If no, then do you recall seeing it in there? If no, did you look? If no, then NOW would be the time to check for it and remove or defeat it..
But If yes,you're sure it's not in there, then;
1) pump the pedal a couple of times then crack the lines at the M/C. If you find pressure there, and the brakes unlock, then the problem is inside the M/C, or the pushrod is too long, blocking the compensating port, or that pesky R-valve .To figure out which, pump the brakes back up, then loosen the M/C from it's mounts, and pull it away from home. If that releases the brakes, then shorten the pushrod.(EDIT;This relaxes the pushrod, and allows the internal spring inside the M/C to push the piston back into the parked position, and that opens the C-port, and that allows the fluid to return, unlocking the brakes.)
2) But if you do not find pressure at the M/C, then
Find out which wheel(s) is/are locked, and crack the bleeder(s),there. If you find pressure in one caliper it's probably a bad hose. If you find pressure in two calipers, it's probably further up the line...... like a residual valve that's not supposed to be on a disc-brake line, or a defective Combination valve but I can't imagine that.
If only the backs are dragging/locked, It's probably a hose, or I suppose a bad Proportioning-valve. But that too would be very rare.
 
Last edited:
71Demon said:
In the master cyl? What is that for? How do you open it?
Click to expand...

You loosen the M/C from it's mount, and withdraw it from the mount. This relaxes the pushrod, and allows the internal spring inside the M/C to push the piston back into the parked position, and that opens the C-port, and that allows the fluid to return, unlocking the brakes.
Oh I guess I accidentally deleted this info.I'll transfer this explanation into post 15.

As to what it's for: is to bring in fresh fluid in compensation for wearing brake pads and shoes. Without it, the pedal would continually drop and over time the pedal would be on the floor.
 
Last edited:
AJ/FormS said:
You loosen the M/C from it's mount, and withdraw it from the mount. This relaxes the pushrod, and allows the internal spring inside the M/C to push the piston back into the parked position, and that opens the C-port, and that allows the fluid to return, unlocking the brakes.
Oh I guess I accidentally deleted this info.I'll transfer this explanation into post 15.

As to what it's for: is to bring in fresh fluid in compensation for wearing brake pads and shoes. Without it, the pedal would continually drop and over time the pedal would be on the floor.
Click to expand...
thanks
 
AJ/FormS said:
Locked brakes and non-returning pedal is fluid not returning into the reservoir, usually a bad hose..... But
did you pull the residual valve out of the port that goes to the front brakes?If no, then do you recall seeing it in there? If no, did you look? If no, then NOW would be the time to check for it and remove or defeat it..
But If yes,you're sure it's not in there, then;
1) pump the pedal a couple of times then crack the lines at the M/C. If you find pressure there, and the brakes unlock, then the problem is inside the M/C, or the pushrod is too long, blocking the compensating port, or that pesky R-valve .To figure out which, pump the brakes back up, then loosen the M/C from it's mounts, and pull it away from home. If that releases the brakes, then shorten the pushrod.(EDIT;This relaxes the pushrod, and allows the internal spring inside the M/C to push the piston back into the parked position, and that opens the C-port, and that allows the fluid to return, unlocking the brakes.)
2) But if you do not find pressure at the M/C, then
Find out which wheel(s) is/are locked, and crack the bleeder(s),there. If you find pressure in one caliper it's probably a bad hose. If you find pressure in two calipers, it's probably further up the line...... like a residual valve that's not supposed to be on a disc-brake line, or a defective Combination valve but I can't imagine that.
If only the backs are dragging/locked, It's probably a hose, or I suppose a bad Proportioning-valve. But that too would be very rare.
Click to expand...
Did not know about a residual valve. Bet thats it. Would that cuase slow stopping too?
 
71Demon said:
Did not know about a residual valve. Bet thats it. Would that cuase slow stopping too?
Click to expand...
No, I think that's a separate issue.
Make sure your reservoir lines are running to the correct ports on the Combination valve. They are supposed to only fit one way on Mopar products, seeing as how the line fittings are different.
The C-valve should be oriented with the P-valve part pointing down; that's the end with the added jut-out. Then:
The radiator-side reservoir should go to the rear drum brakes, via the lowest side-port on the C-valve.
The firewall side reservoir should be plumbed to the top-most end-port which should immediately split off to the left and right calipers

I get the feeling your front brakes are not working correctly,if at all. Your pedal, on application, should almost immediately be hard, within about 1 inch of travel. I call this a high,hard,pedal.

We had one fella here, with a Scarebird set-up,
that had an insufficient-front-brake application that he traced to;the M/C did not have enough stroke to properly energize the calipers.
Well if you study that out;It shouldn't be possible unless there is something wrong in the rear system or the pedal-ratio. None-the-less, his solution was a bigger M/C bore with more fluid capacity.
__________________________________________________
If you study out the M/C, you will see that while you are pushing the pedal down, you are sliding the front-brake piston down the bore. If the rear brakes are working correctly, then the rear-brake piston will also be sliding down the bore, being pushed along hydraulically,by the brake fluid trapped between the pistons. The front brakes will not be energized until the rear-brake piston stops moving, and has taken up all the slack in the rear slack-adjusters, and the shoes are touching the drums.
Then, with everything now hydraulically locked up, the fluids have no place to go but out towards the calipers and wheel-cylinders. So the pedal gets hard, and that's when pedal-modulation can begin.

Now, if something is mechanically wrong in the rear-brake system, that the piston slides too far down the bore, then the front-brake piston will follow it, also too far. The brake pedal, of course will also fall lower than expected. But finally Hydraulic lock is achieved and braking can begin. But the pedal is nearly on the floor, and there is only a fraction of fluid left in the bore to be pushed out towards the calipers. If there is even the slightest thing wrong in the front system; like the pistons being knocked back, the sliders not working properly,flex in the pads or hoses,or even a tiny pocket of air; then there may be insufficient reserve capacity for the front brakes to be properly energized.
Now if the hydraulic system has failed in the front brake system, or if for some reason the pedal travel is too great; then the front brake piston will travel down the bore until it hits the rear-brake piston, and then, with a very low pedal, the rear brakes can still be modulated . However, the rear brakes do not receive full line-pressure, but only a fraction of it,and the the rear-brake sizing is purposely kept weak to prevent rear-wheel lock-up, so taken together,you have to apply a lot of leg pressure to get any slowing down of the car, which you, the operator, interpret as "poor braking". Very poor.

But..... if you have a high,hard, pedal almost immediately on pedal application....then none of this applies; none of it.
 
Last edited:
I had the same issue after a front disc upgrade. A 15/16 bore mc fixed it. No power booster.
But def call Dr.Diff.
 
AJ/FormS said:
No, I think that's a separate issue.
Make sure your reservoir lines are running to the correct ports on the Combination valve. They are supposed to only fit one way on Mopar products, seeing as how the line fittings are different.
The C-valve should be oriented with the P-valve part pointing down; that's the end with the added jut-out. Then:
The radiator-side reservoir should go to the rear drum brakes, via the lowest side-port on the C-valve.
The firewall side reservoir should be plumbed to the top-most end-port which should immediately split off to the left and right calipers

I get the feeling your front brakes are not working correctly,if at all. Your pedal, on application, should almost immediately be hard, within about 1 inch of travel. I call this a high,hard,pedal.

We had one fella here, with a Scarebird set-up,
that had an insufficient-front-brake application that he traced to;the M/C did not have enough stroke to properly energize the calipers.
Well if you study that out;It shouldn't be possible unless there is something wrong in the rear system or the pedal-ratio. None-the-less, his solution was a bigger M/C bore with more fluid capacity.
__________________________________________________
If you study out the M/C, you will see that while you are pushing the pedal down, you are sliding the front-brake piston down the bore. If the rear brakes are working correctly, then the rear-brake piston will also be sliding down the bore, being pushed along hydraulically,by the brake fluid trapped between the pistons. The front brakes will not be energized until the rear-brake piston stops moving, and has taken up all the slack in the rear slack-adjusters, and the shoes are touching the drums.
Then, with everything now hydraulically locked up, the fluids have no place to go but out towards the calipers and wheel-cylinders. So the pedal gets hard, and that's when pedal-modulation can begin.

Now, if something is mechanically wrong in the rear-brake system, that the piston slides too far down the bore, then the front-brake piston will follow it, also too far. The brake pedal, of course will also fall lower than expected. But finally Hydraulic lock is achieved and braking can begin. But the pedal is nearly on the floor, and there is only a fraction of fluid left in the bore to be pushed out towards the calipers. If there is even the slightest thing wrong in the front system; like the pistons being knocked back, the sliders not working properly,flex in the pads or hoses,or even a tiny pocket of air; then there may be insufficient reserve capacity for the front brakes to be properly energized.
Now if the hydraulic system has failed in the front brake system, or if for some reason the pedal travel is too great; then the front brake piston will travel down the bore until it hits the rear-brake piston, and then, with a very low pedal, the rear brakes can still be modulated . However, the rear brakes do not receive full line-pressure, but only a fraction of it,and the the rear-brake sizing is purposely kept weak to prevent rear-wheel lock-up, so taken together,you have to apply a lot of leg pressure to get any slowing down of the car, which you, the operator, interpret as "poor braking". Very poor.

But..... if you have a high,hard, pedal almost immediately on pedal application....then none of this applies; none of it.
Click to expand...


Thanks. I have plumed it 2 ways, with front(radiator side) of the MC going to the front brakes(per wilwood tech) and then the firewall going to the back brakes then I reversed them as factory. Are wilwood mc's different or are they confused? Lot of good info here,thanks for taking time.
 
67dartgtgo said:
I had the same issue after a front disc upgrade. A 15/16 bore mc fixed it. No power booster.
But def call Dr.Diff.
Click to expand...
Do you have 4 wheel disc? Mine stopped good until I added rear discs.
 
You must log in or register to reply here.
-

Back
Top