Disc conversion question

[tjdriscoll]

Hi Folks,
I need a sanity check before I make any more modifications so I am asking the collective brain-trust. I converted the front brakes on my 68 Charger from the stock 10” power drums to disc. I’ve kept and rebuilt the rear drums.
Vehicle Specs:

• 68 Charger
• 440cui V8
• Stock cam
• Makes 17-18” vacuum
• 8” Bendix Style dual-diaphragm brake booster (new OEM-style replacement)
• All new stainless OE-style brake lines
• Wilwood 1 1/8” master cylinder
• Wilwood 11.75” Front disc brake conversion kit (uses original drum spindles)
• OE 10” rear drum brakes
• 1-pc brass OE-style proportioning valve (purchased from Dr. Diff)

My question:

Wilwood recommended I install a 10-psi residual pressure valve for the rear drums in order for both the discs and drums to engage at the same time. My understanding is that I install the pressure valve after the proportioning valve to the line running to the rear drums. Is this correct? Do I even need the residual pressure valve at all if I’m using the OE-style proportioning valve?

 

[MoparMur]

No expert here but here's my 2 cents. I have seen 1 or 2 lb. check valves installed in the rear line after the brake proportioning valve. This is installed to keep a minimum pressure in the line to keep the pistons in the wheel cylinders slightly applied to keep the shoes just contacting the drum, so you don't need to pump the pedal one or two times to get the shoes out to the drums. Never have I heard of a 10 lb. valve in the line! Maybe you had a typo there and meant to say 1 lb.? To me if there were 10 lbs. pressure there I'd think the brakes would be applying without pedal input. Perhaps I'm wrong and others will chime in?

 

[Kern Dog]

Either you misunderstood or you talked with an employee that was planning to quit that day and was messing with you......

You don't need that crap.
I have disc/drum cars without any residual pressure valves anywhere and they brake quite well.
I just did a booster swap on a 68 Plymouth with the Wilwood front brakes, stock type 10" rear drums and it stops with amazing feel and response.

No residual pressure valves in the car and none are needed. It has a proportioning valve though.

 

[steve340]

From what I know:
Willwood recommend a 10 lb valve if the master cylinder is lower than the braking components - like on a 1934 Ford where the master cylinder is under the floor.
The discs will not need a residual valve.
If you have an OE one in the master no need to fit another to the rear drums.
The OE proportioning valve does not have residual valves in it - they were located in the master cylinder originally.

 

[R/T Boy 67]

When the master cyl. is above the wheel cyl. then the only reason for the residual pressure valve is to stop weeping at the wheel cyl. They installed expanders, or those big conical springs in the cylinders to keep the seal tight against the bore, decades ago, and the valve was not needed after that. It has nothing to do with the rear brakes getting a head start, they used a metering valve for that.

 

[Sixpactogo]

I too have a 68 Charger that I converted from front drums to stock mopar discs. My car stops great with just the correct Proportioning valve. IMO, if you're happy with the system as is, I would not buy into any added gizmos. On another note,You mentioned you had 10" drums on the front. Was it a small block car? My car was a 383 car with 11" front drums.

 

[Thrashingcows]

If you are concerned at all just install an adjustable pressure valve between the master cylinder and the rear drums, then you can dial it in exactly how you want it.

 

[tjdriscoll]

I too have a 68 Charger that I converted from front drums to stock mopar discs. My car stops great with just the correct Proportioning valve. IMO, if you're happy with the system as is, I would not buy into any added gizmos. On another note,You mentioned you had 10" drums on the front. Was it a small block car? My car was a 383 car with 11" front drums.

My car was always a big block. Started life with a 383 and 3-speed auto. Someone converted it to a 440 4-speed before I bought it. It came with the 10" drums all around.

 

[tjdriscoll]

Either you misunderstood or you talked with an employee that was planning to quit that day and was messing with you......

View attachment 1542943

You don't need that crap.
I have disc/drum cars without any residual pressure valves anywhere and they brake quite well.
I just did a booster swap on a 68 Plymouth with the Wilwood front brakes, stock type 10" rear drums and it stops with amazing feel and response.

View attachment 1542945

No residual pressure valves in the car and none are needed. It has a proportioning valve though.

Yeah that was my gut feeling too... But this is the first time I've attempted brakes. I have the correct proportioning valve so I'll just connect it as suggested.

This was what they sent me:

Hello,

Thank you for the inquiry with Wilwood Disc Brakes.

We’re happy to help you find a master cylinder for your build , however we need a bit more information about your build in order to make an accurate recommendation.

If you’re using power brakes, are you making at least 18” of engine vacuum to the booster at idle (with no vacuum pump)? Are you using at least an 8” dual diaphragm booster?

With our master cylinders you’ll need an adapter plate from a company called Mancini racing-(586) 790-4100 as well as a 10lb residual pressure valve in the rear line while you have the drum brakes in the rear.

If you’re interested in switching to manual brakes, do you know your pedal ratio? If you are unsure, it’s very easy to measure. The link below contains our pedal ratio guide which should help you determine your pedal ratio:

 

[Kern Dog]

That looks like a generic tech sheet that goes far beyond the needs you'd encounter.
I've driven cars with less than 18" of vacuum and they stop fine.
I've never used residual pressure valves in any of my projects and have had cars that stop better than new cars.
Wilwood was playing it super cautious with the wording in that announcement. I understand the importance of safe braking but they are exaggerating.

 

[Sixpactogo]

My car was always a big block. Started life with a 383 and 3-speed auto. Someone converted it to a 440 4-speed before I bought it. It came with 10" drums all around.

Maybe I got some wrong info somewhere along the line. Anyway, I was led to believe all big block Chargers got the 11" front drums or optional disc's and 10 rears. Mine was a 383 auto also but had 11" front drums. Another one of those "never say never" adages.