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Help with Rear Drum nightmares 66 Coupe Power vs Manual Brakes

cturboaddict

New Member
Hey guys, reaching out for some help here.
I have a 66 Mustang, I did a CSRP power front disk conversion a while back and I'm finally on the road.
I'm having some braking issues, and was hoping someone may have some direction to point me into.

#1- I built a 331 Stroker with Sniper EFI, and a 4r70w trans. My Lunati cam is only pulling 12 at idle, and the brakes are lackluster. At speed (say 40mph) when you come off the gas and the engine goes to full vacuum, it only takes a tiny touch and the brakes are activated. At a stoplight or pulling out of a parking spot with no throttle, they are no where near as reactive. Should I try a booster or should I just swap to a manual wilwood brake?

#2- I've had nothing but a horrible time with the rear drums. Once I got it on the road, the rear drums didn't work at all. I couldn't jack up the rear and put it in gear and have the rear tire stop. They would just spin through the braking. The previous owner just slapped something on when I bought the rolling chassis, and I found the brakes to be a disaster. I pulled them completely off to the wheel cylinders and replaced with new cylinders, drums, pads and hardware. The drums/pads/hardware came as a kit from RockAuto on the Powerstop brand. Now, I've adjusted them multiple times. I've rechecked everything with the installation. But when I apply any type of moderate brake, the rear drums shudder so hard and shake the heck outta the back of the car.

Any thoughts on #1, and any direction on #2 is appreciated. I'm at my wits end with the drum brakes before I pull them off and just buy a rear disc conversion.
Thanks guys!
 

stangfbank3503

New Member
First thing that comes to mind is if you have a proportioning valve in the system? If not I’d get an adjustable one and start playing with the setting on it and that should fix the problem with the rears.

The front I would try a booster or maybe even add a vacuum canister possibly.

Hope that helps


Sent from my iPhone using Tapatalk
 

cturboaddict

New Member
First thing that comes to mind is if you have a proportioning valve in the system? If not I’d get an adjustable one and start playing with the setting on it and that should fix the problem with the rears.

The front I would try a booster or maybe even add a vacuum canister possibly.

Hope that helps


Sent from my iPhone using Tapatalk
Yes, I have an adjustable prop valve. I've been tightening the fronts more and more, but it's not seeming to make a difference.
I'll keep at it.

The front stops, it's just lackluster at idle. The pedal feel is not consistent which bothers me more than anything. If they just stunk all the time, i maybe wouldn't complain so much. But sometimes they bite, and other not, so that drives me crazy.

Hence why I thought of ditching the power assist and just putting in a manual wilwood master. Spoke with wilwood and sized it appropriately already. Sure it's another $350 into the brakes, but I need safety and predictability.
 

Horseplay

I Don't Care. Do you?
As simple as brake systems are it can be very difficult trying to diagnose stuff remotely just going off what someone thinks to share. Bore size of the master, size of the brake lines themselves and a myriad of other things can themselves or in combination contribute to issues. That said, some thoughts.

First off, you don't need power brakes in such a small light car especially when using front discs. Using the correct pedal geometry and bore sized master my 65 seems to brake just as well as my old '14 GT (full Wilwood all around). Pedal effort is a bit more than power assisted set-ups but I can skip leg day and stop just fine. If your engine isn't providing the necessary vacuum for a booster consider going manual. Vac canisters and other band aids never seem to work out in my experience.

A proportioning valve is used to limit line pressure to the rear system in comparison to the front in a disc/drum set-up so the rear shoes do not lock up under hard braking. If you do not have one installed you should. Mandatory equipment with any disc/drum system. Could your description of the rear brake action be them starting to lock-up due to excessive line pressure? Could be. Like I wrote, its hard to decipher online messaging into real life sometimes. If you a sure everything is right in the set-up, maybe consider a part issue. It happens where drums are not perfectly machined (round) out of the box, for example. Most parts houses will turn drums for you. Have them put them in the machine and check them for "trueness" by turning them a bit.

Rear drums are perfectly fine for any type of driving short of actual racing (and for decades they were just fine there too) especially when you have front disc. Those do most of the work anyway. BUT...Wilwoods rear kit with the integral parking brake is a really nice upgrade!

I have to ask only because of how often this is the real reason for most brake issues. Are you absolutely positive you bled the system in it's entirety completely? Really? Pedal is very firm at all times? Unless you are positive, I suggest bleeding it all again, starting passenger rear then D rear, P fronts and then D front last. It doesn't take much air at all in the system to cause all kinds of issues. If you didn't bench bleed the master before installation you almost certainly have/had some air trapped in it that worked its way in the lines. The best way to bleed brakes in my experience (and I had quite a time when first doing my 65) is to use the old vac hose on the bleeder into a half filled bottle of brake fluid method. This way you can do it yourself and be assured no air is slipping back in on pedal release. Just be sure to put a little grease around the bleeder threads for extra peace of mind.

That's all I got that comes to mind.
 

cturboaddict

New Member
As simple as brake systems are it can be very difficult trying to diagnose stuff remotely just going off what someone thinks to share. Bore size of the master, size of the brake lines themselves and a myriad of other things can themselves or in combination contribute to issues. That said, some thoughts.

First off, you don't need power brakes in such a small light car especially when using front discs. Using the correct pedal geometry and bore sized master my 65 seems to brake just as well as my old '14 GT (full Wilwood all around). Pedal effort is a bit more than power assisted set-ups but I can skip leg day and stop just fine. If your engine isn't providing the necessary vacuum for a booster consider going manual. Vac canisters and other band aids never seem to work out in my experience.

A proportioning valve is used to limit line pressure to the rear system in comparison to the front in a disc/drum set-up so the rear shoes do not lock up under hard braking. If you do not have one installed you should. Mandatory equipment with any disc/drum system. Could your description of the rear brake action be them starting to lock-up due to excessive line pressure? Could be. Like I wrote, its hard to decipher online messaging into real life sometimes. If you a sure everything is right in the set-up, maybe consider a part issue. It happens where drums are not perfectly machined (round) out of the box, for example. Most parts houses will turn drums for you. Have them put them in the machine and check them for "trueness" by turning them a bit.

Rear drums are perfectly fine for any type of driving short of actual racing (and for decades they were just fine there too) especially when you have front disc. Those do most of the work anyway. BUT...Wilwoods rear kit with the integral parking brake is a really nice upgrade!

I have to ask only because of how often this is the real reason for most brake issues. Are you absolutely positive you bled the system in it's entirety completely? Really? Pedal is very firm at all times? Unless you are positive, I suggest bleeding it all again, starting passenger rear then D rear, P fronts and then D front last. It doesn't take much air at all in the system to cause all kinds of issues. If you didn't bench bleed the master before installation you almost certainly have/had some air trapped in it that worked its way in the lines. The best way to bleed brakes in my experience (and I had quite a time when first doing my 65) is to use the old vac hose on the bleeder into a half filled bottle of brake fluid method. This way you can do it yourself and be assured no air is slipping back in on pedal release. Just be sure to put a little grease around the bleeder threads for extra peace of mind.

That's all I got that comes to mind.
Thanks for the thorough response.

Correct, I'm considering the switch just for consistency with my vacumm at idle issue. I've spoken with Wilwood and they receommended the appropriate size master cylinder. Which would also be the same if I switch to rear disc later. (Just need a residual valve for the current drums)

Yes, I have a proportioning valve.

I don't think it's them starting to lockup. It's like they grab/release/grab/release/grab/release at an extremely fast pace depending on vehicle speed. Light pedeal pressure, and I mean REALLY light pedal pressure stops the car slowly and smoothly. You add foot pressure to a moderate type stop and the back end shudders like crazy. (Yes, I understand it's hard to show/describe via words... I should just make a video)

I'm starting to wonder about the part issue. You can see the serrations on the actual pad to what looks like the shudder to me (i'll post a photo here in a second). As noted, the drums are brand new, but I'm curious as to the fitment of the shoe to the drum. I don't think the radii are exactly the same of the outside radius of the new shoes, to the inside radius of the drum.

I've bled them countless times. First master cyl before install. Then pressure bled the system. Then followed through with bleeding via the old school pump and crack them open. I use a one man bleeder anti siphon catch can. Similar to your mentioned method, but with a check valve in the line.

I'll check the parts and the bleeding again and report back. Photo incoming.
 

Starfury

Well-Known Member
I put manual CSRP discs on my 67 knowing that I'm only pulling 10-12" at idle, and I do not regret it. They stop a hell of a lot easier than the manual drums ever did, and I much prefer the predictable travel and feedback through the pedal to a boosted setup.

Regarding the rears, it kind of sounds like they're not installed correctly. Another option is that the drums are actually out of round. I used to see that a lot when I machined drums. Might be worth taking the drums to a shop to have them put on a lathe.
 

cturboaddict

New Member
I put manual CSRP discs on my 67 knowing that I'm only pulling 10-12" at idle, and I do not regret it. They stop a hell of a lot easier than the manual drums ever did, and I much prefer the predictable travel and feedback through the pedal to a boosted setup.

Regarding the rears, it kind of sounds like they're not installed correctly. Another option is that the drums are actually out of round. I used to see that a lot when I machined drums. Might be worth taking the drums to a shop to have them put on a lathe.
Will do. I'm leaning towards swapping to manual for certain. Pedal effort a nightmare? I hear its nothing from alot of people.
 

Starfury

Well-Known Member
I was worried when I did it that it would be really heavy, but it's a lot lighter than the manual drums ever were. It's perfectly driveable, and not something I'd worry about unless you have bad knees.
 

Horseplay

I Don't Care. Do you?
The key to a good manual system is to correctly mate the master bore size to the pedal ratio. The mechanics of the system dictate the "feel" and muscle force necessary to make it all function.
 
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