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Hi, I have disc brakes up front and drums in the rear of my 52 suburban street rod, and I have been having issues getting them to work right. The from brakes came with a Mustang II kit from fatman (they are Gm calipers and ford discs). I am running a dual master cylinder/booster designed for a disc/drum combo and an adjustable proportioning valve from wilwood.
Everything in the system is brand new. I initially had a spongy pedal after bench bleeding and bleeding the entire system multiple times. So I ajusted the pushrod on the booster and the pedal firmed up... but later developed brake drag after driving for more than 5 miles. So I backed off the pushrod, to once again have a spongy pedal. When I plug the ports in the master cylinder the pedal is completely stiff, so I don't believe there is a problem with the master cylinder. I have checked the lines for leaks, and the level in the master cylinder does not drop, so I don't think that is an issue. I have also bled the system problem 15 times, both with the two person method and with a vaccum unit running off of my compressor.
Today I decided to plug just the rear port and the master cylinder and then also just the front port. When plugging the feed to the front brakes the pedal was stiff. But when nearly to the floor when plugging the port to the rear. This leads me to believe that the problem is with the front brakes. So I put a tube on the front brakes and had my dad push the pedal in with the bleeder cracked. No fluid came from the tube until the pedal was nearly to the floor. Does anyone know what could be causing this? Like I said, the pedal was almost instantly stiff with connected to only the rear brakes. Its like the two ports in the master cylinder are activating at two different times. Any help would be greatly appriciated. thanks

Phil, I assume if it's an underfloor set-up you've got the lines installed in the M/C correctly?
You seem to have covered all the other bases.

Bear in mind if it is underfloor, the M/C is reversed relative to its original position on a firewall.
Is it possible Fatman have sent you a disc/disc M/C?
AFAIK discs require much less volume of fluid to operate.

The front circuit should activate slightly before the rears as they do most of the braking.

"The front circuit should activate slightly before the rears as they do most of the braking."

Although I agree the front does most of the braking... The rear brakes are actuated before the front brakes to help pull the vehicle straight.

The rear brakes are usually the circuit that is closest to the pushrod. pressure builds up here first before actuating the other circuit.

Make sure your adjustment between the master cyl. and booster is correct. To long, you have brake drag and to short, to much pedal drop. Put a vacuun gauge on your intake, start rod up and get a reading. Depress brake pedal and you should see a momentary drop in vacuum. If vacuum remains low with depressed pedal then booster is leaking. I have the same setup and had the same problems. Finally took Hotrod Lincoln's advice and gravity bleed the front. This means I jacked up the rear on six ton jackstands to get the MC higher than the front bleed screws. Good luck

thanks for all the advice. Its actually a firewall mounted master cylinder (I believe its out of a 78 fairlain), if that makes any difference. The pedal is sloppy regardless if vacuum is applied to the booster or not.

oh, the the larger reservoir is closest to the pushrod (which is the reservoir I have attached to the discs up front)

Had exactly the same issue when I switched out my M2 rotors for larger Granada rotors & GM metric calipers. Could not get any back pressure whatsoever ! It turned out to be a combination of issues which separately likely wouldn't have caused a problem, but together screwed things up. First, make sure your rear shoes are adjusted up properly. I changed Master Cylinder to a larger bore ( F 250 MC ) as GM calipers seem to need a little more volume. I also tweaked the booster piston a bit, although I had same issue with brakes seizing with too much adjustment. Also check the connection point between your brake pedal arm and the booster rod for too much slack.
Good luck !

thanks Bruce. What year F250 master cylinder did you use. I had been thinking that maybe the master cylinder wasn't getting enough volume to the calipers.

It was a Raybestos # MC36468

Also, Bruce, did you bolt that master cylinder up to the booster that you had previously, or did you have to get a different booster for the f250 master cylinder?

Oh, one more thing, I looked up that part number and it didn't show it fitting an F250. Are you sure it wasn't MC39468 (that fits an f250). Just want to make sure I fit a master cylinder that has worked for somebody with a similar setup. thanks

Hy PhilBurban, I helped britcon with this project, the master cylinder [Raybestos 36468] is a 1 1/8" bore designed for a 1976 Tbird with four wheel disc brakes. Britcon has a residual pressure valve, and an adjustable proportioning valve in the rear circuit. The master cylinder mounted to the same booster as the smaller bore one it replaced. Part of the problem with britcons system was that his brake pedal could not go all the way to the floor because of interference with the steering column, hope that helps.

If that's the case i've been mis-informed,I stand corrected.

Sorry PhilBurban......

It's the other way around. 99% of vehicles with a split system have the front brakes on the pushrod end, rear brakes on the other. Many times, the ports are different sizes just so they can't be swapped the wrong way.

*G*....what Bobb ( 3B ) says. I pulled that part # off the invoice so it is correct. I was sure Bobb told me it was for an F250 - definitely a larger MC than the T-Bird one I had originally. At any rate, without Bobbs' expertise, I would still be pumping that pedal !

thanks guys. So if I use that master cylinder, which is designed for disk/disk, will I need to do anything to my system to accommodate for the drums in the rear?

Ok, I'm still having problems with my power brake conversion. The pedal still goes way too far down to the floor.
Beltfed says to check the adjustment between the m/c & booster. How do you do that ? I thought the rod is a fixed length.

nvrdone. Separate the master cylinder from the booster. Grip the pushrod with some pliers and turn the nut on the end. Turning it out will cause the brakes too activate sooner. Turn it too far though and you will have brake drag. Make sure to leave a little freeplay in the pedal.

89.7% of all statistics are just made up!

Maybe it doesn't make that big of a difference. The CPP MC kits for our trucks are set up with the rear brakes closer to the pushrod. On the corvette style MC, the circuit closest to the pushrod actuates the other circuit after pressure has been built up. There is a mechanical interlock that will actuate the second circuit if the pressure hasn't built up.

Maybe some MCs use a solid link between the 2 circuits making this timing less critical. Combination valves can also provide a slight delay.

I don't have the time or the resources to investigate every MC setup, but the MCs that have been specefically setup by the aftermarket for our stovebolts use the circuit closest to the pushrod for the rear brakes.

I've put in my fair share of master cylinders over the last 20 years, all of them were plumbed with the pushrod end going to the front brakes. These are mostly disk/drum or disk/disk vehicles, including a few Corvettes.
You are probably corrct about the aftermarket master cylinders being specifically set up different from an oem style.

Hy PhilBurban, are you plumbing a combination/metering valve into your system? If you are, make sure it is for a disc/drum system, if you're not going to use that valve then you will need a 10 p.s.i. resisdual pressure valve in the rear brake circuit, and possibly an adjustable proportioning valve.

I don't want to hijack this thread, but here are a couple helpful links from CPP showing cutaways and plumbing of the booster, mc, and proportioning valve. All the components need to be able to play well together to have safe braking. I assume CPP has somebody working for them that is smarter than me that "engineered" all these components to work well together.

http://www.classicperform.com/Instructions/PDF/PVK.pdf
This link shows the plumbing of the corvette style MC with the combination valve.

http://www.classicperform.com/tech_articles/PDF/Dems_DA_Brakes_3.pdf
This link has some nice cutaways, if you ever wondered what was inside.

There are many things to consider when installing a brake system. Improper plumbing could be responcible for loss of control in a panic stop, so it is important to know how every component works, independantly and collectively if one chooses to "reinvent the wheel".

Here's another link to M/C plumbing about what goes where.

My master cylinder is quite a bit higher up than all of my brakes, so I didn't install an residual pressure valves. I put in the new master cylinder today, and things were a bit stiffer, but not as great as I had hoped. So I took of the calipers and tilted them so that the bleeder valve was straight up and sucked some of the fluid out, hoping to get any trapped air. That stiffened them up a bit more. I still think I need to adjust the rear brakes a little better. I also realized that one of my (re-manufactured) wheel cylinders is leaking just a little bit, so after replacing that I'll have to see what happens. Anybody else have any other suggestions?

Hy PhilBurban, for years and years drum brake master cylinders have had resisdual pressure valves built into them, they are there to keep the drum brake wheel cylinders "charged", so that no part of the pushrod stroke is waisted "recharging" the wheel cylinder. I've said it before and I'll say it again, I'm not an engineer, and I won't modify a proven and safe design. A GM engineer once said "parts left out cost nothing, and cause no service problems, so you can be sure if we put it in there we feel it is necessary." Resisdual pressure valves have been in every drum brake master cylinder I have ever worked on, regardless of the mounting location on the vehicle, they are also used in dual systems with disc/drum configurations, and 2 p.s.i. resisdual pressure valves are specified in disc/disc systems where the master cylinder is mounted below the level of the brake calipers.

Thank for the good info 3B. I noticed that the master cylinder I got (the 76 T-bird one) was also used in some cars with disk/drum setups. Does that mean it has a residual pressure valve build in (because it supports drums), or would that have been build into the combination valve?

Hy PhilBurban, if they are listing one master cylinder for both disc/disc and disc/drum systems then there probably are no resisdual pressure valves in the master cylinder itself, the resisdual pressure valves would probably be in the correct combination valve for the disc/drum application, hope that helps.

alright, thanks 3B. Looks like I better add a residual pressure valve then just to be safe.

I wonder if brake line size has anything to with response time, most rear brakes use 1/4" line whereas disc brake utilize alot of 3/16" line, what is your m/c plumbed for?