Which wheel cylinder?

[///Matt]

hey guys,

I’ve got a ‘68 Satellite, whcih I believe was originally an 8 1/4 rear but presently has an 8 3/4 rear. I’m not fully certain which drums are back there, either 10.5 or 11”. Not clear where that measurement is taken to decide which they are.

I converted the front to a wilwoood disc kit, and put a Dr Diff 15/16” MC on it, eliminated the factory distribution block, and a wilwood adjustable proportioning valve in the rear circuit.

I have not added an RPV to the rear circuit yet, and it’s evident when first braking, as I have to take up slack from the rear before the brakes really grab.

I also cannot bleed the rear properly, because the bleed hole in the cylinders are jammed up with something.

So I need new wheel cylinders, but I don’t know for sure which ones I need. Got any insight for me?

I’m told that there exist cylinders I can use which eliminate the need for an RPV, as they are essentially integral to the cylinder.

Whatcha got for me?

 

[R/T Boy 67]

New wheel cyl. do not require a RPV. Your brakes are either 10 or 11 inch. Measure the inside diameter of the drum. Slack in the rear drums is an adjustment problem, or air in the rear system.

 

[PurpleBeeper]

Yep, what R/T boy 67 said...measure the inside diameter of the drums to determine the size. If the wheel cylinders aren't leaking & you can get the bleeder screws loose, replacement bleeder screws are available at most part stores.

 

[///Matt]

I can take the bleeder screws out entirely still not get air or fluid from the cylinder. The bleed hole on both is gummed with some combination of junk, paint, and rust. I can literally remove the bleed screw and actuate the brakes without losing fluid... so they’re getting replaced no matter what.

Are the cylinders different betweeb various drum sizes in these axles? I would go to summit and search by “68 satellite” but since they’re likely not even from a 68 I don’t know if I’ll get the right ones.

 

[steve340]

I think most originals rear cylinders seem to use 15/16. From what I can recall the 10 and 11 use the same size.

 

[///Matt]

I think most originals rear cylinders seem to use 15/16. From what I can recall the 10 and 11 use the same size.

Same cylinder, or just the same bore size? Like mounting locations and all the same?

 

[PurpleBeeper]

I think Steve340 is right, but just check for part number differences on autozone.com or something to be sure. I think the wheel cylinder(s) itself is the same part for either drum brake size... and flush the heck out of that brake system if it's that bad.

 

[Hemi 354]

I found that Autozone and O'Reilly's must sell Chinese junk. When I replaced mine, I had on hell'va time with the bleeder screws, fitment of the cylinder, etc. I returned them and went to NAPA. Problem solved.

 

[Billccm]

I have a front disc conversion, portioning block, and 15/16 master all sourced from Dr Diff. I replaced the rear wheel cylinders with Raybestos stock replacement parts for a 68 Road Runner. My rear brakes will try to grab and lock right near the stop from a 40MPH pedal stomp. Car is stable. I'm happy.

 

[steve340]

If you want to reduced the amount of force a wheel cylinder can apply you go smaller bore size.
I have 11 inch heavy duty rear drums with stock front discs on a Challenger. To be sure my car would not lock the rears I had the wheel cylinders sleeved to 3/4 and it has worked out good.
I can slam on the brakes with not a hint of rear lockup. I am still not brave enough to slam them on in the wet - maybe one day I will need to in an emergency.
The weight of the car, how the weight transfers to the front and the quality of the tyres all come into play with regards to rear wheel lockup.
This is just my opinion - if you have the 10 inch drums try the standard wheel cylinder and go from there.
Your 15/16 master cylinder is generating more line pressure than a standard 1 or 1&1/8 inch would have feed down the lines. This may make the rears tend to lock sooner as well.
Get the brakes bled and all operating correctly first. Once they are working right you can make a good assessment of what you may need to change.

 

[R/T Boy 67]

Listen to Steve 340. He is right on.

 

[///Matt]

I think Steve340 is right, but just check for part number differences on autozone.com or something to be sure. I think the wheel cylinder(s) itself is the same part for either drum brake size... and flush the heck out of that brake system if it's that bad.

I disconnected the T at the differential and flushed all the old fluid out back to there, but unable to bleed at the wheels yet... once I get the new cyls on it will get wellllllllll bled.

 

[///Matt]

If you want to reduced the amount of force a wheel cylinder can apply you go smaller bore size.
I have 11 inch heavy duty rear drums with stock front discs on a Challenger. To be sure my car would not lock the rears I had the wheel cylinders sleeved to 3/4 and it has worked out good.
I can slam on the brakes with not a hint of rear lockup. I am still not brave enough to slam them on in the wet - maybe one day I will need to in an emergency.
The weight of the car, how the weight transfers to the front and the quality of the tyres all come into play with regards to rear wheel lockup.
This is just my opinion - if you have the 10 inch drums try the standard wheel cylinder and go from there.
Your 15/16 master cylinder is generating more line pressure than a standard 1 or 1&1/8 inch would have feed down the lines. This may make the rears tend to lock sooner as well.
Get the brakes bled and all operating correctly first. Once they are working right you can make a good assessment of what you may need to change.

Thanks Steve. Looking to put stock-ish cylinders back on, just need to be sure I get the right ones!

 

[69L48Z27]

You would adjust the propotion valve to make the rears do not lock up. I like using the Rock Auto catalog for seeing all the variances in parts available, and sizes.

 

[bc3j]

I have Mopar sourced front disc and stock 11” rear drums with 15/16” rear wheel cylinders. I used the factory disc brake combination valve (used after Jan 1, 1970) with the metering valve. I have manual brakes with a 15/16” MC and do not lock my rears on heavy braking. I don’t use an adjustable proportioning valve. It’s on an E body and works ok for me. My brake system is entirely stock Mopar parts. Hope this might help.

 

[idrivemopar]

If you have taken the bleed screws out and nothing still comes out and you say they are gunked up with something, its most likely rust in the cylinder and if that is true, the bores are most likely pitted where they have rusted. It is a good idea and inexpensive to replace them. Also, be aware, there are many Chinese versions of brake cylinders out there that do not have the correct size bleeder ports. They are typically smaller, and not fun to try and bleed. I would suggest taking yours with you to make sure you get ones that have properly sized bleeder screws and ports. I think I mentioned this in my build thread or another several years ago in this forum. Just be aware!

 

[///Matt]

Cylinders are definitely getting replaced! Just want input on what care I may need to take to get the right ones

 

[idrivemopar]

Here is the thread I started a ways back, in case you are interested in reading about the issues with undersize bleeders.
https://www.forbbodiesonly.com/mopa...ke-them-with-correct-diameter-bleeders.62873/

 

[MW 413]

Make sure your MC outlet port to the rear drum brakes has the rubber check valve and spring in it. This has to be there to hold 2psi residual line pressure so the rear wheel cyls have cups expanded and don't require priming with each pedal hit. You will have a low pedal with the first pedal hit each time if its missing. If the MC was set up for 4 wheel disc brakes it is definitely missing. Disc brakes do not require constant residual line pressure between pedal hits.