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Rear brakes locking up after front disc conversion

14K views 57 replies 20 participants last post by  reevescajr 
#1 ·
Hey friends I recently did a wilwood front disc conversion and a duel bowl upgrade. I took the car out and have run into and issue when braking the rears like to lock up while heavy or even light braking. I am guessing I need an adjustable purportioning valve. Right now the two lines out on the duel bowl go into a brass splitter and then off to the front and off to the back. Any advice on if I should go with an adjustable value and what one time go with? Thanks guys
 
#2 ·
Be very careful driving that car, its quite dangerous when the rears lock first especially if you are not going exactly straight. Where did the brass splitter come from, if its the stock one, it has to go! You need one made for disk/drum brakes from a Mustang or similarly balanced coupe, same with the master cylinder. If the master was not specifically designed for disk/drums, then you need a residual valve for DRUM brakes plumed into the brake line, but that will not fix your problem, it will just give you better pedal height. If you get a "splitter" from a later coupe e.g. 80s, they have a delay circuit for the rears to insure they do not lock up while the weight gets thrown on the front tires, I used one from an 86 T bird coupe, love it.



Before you head for a proportioning valve, what you have is not working correctly unless you added disks in the rear then all you need is a proportioning valve. Duplicate a stock brake "system" for a disk/drum vintage mustang.


Just out of curiosity, are you sure you got the right fitting on the master going to the fronts?


Good luck
 
#5 ·
That master brake cylinder looks like a drum/drum one not a disc/drum. Usually the disc/drum has the rear cylinder larger as the front disc brakes require more fluid to work than drums. But I think you need a proportioning valve to push less pressure to the rear then the fronts.
 
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#7 ·
Yes after more looking it is a drum/drum mc. I think tho with the value it will remedy things. I'm getting good strong breaking on the front disc.

The way I see it is if the larger reservoirs in the MC to push more fluid to drums are pushing towards disks it just has to work less than it would have before. That's my theory to cover up that I got the wrong one lol
 
#8 ·
I used the exact same kit and a wildwood style dual bowl on my front disk conversion. +1 to @dobrostang and his recommendations. I bought a brass T fitting from Ace hardware that I plumbed my front brakes through that. I routed the rear brakes through an adjustable proportioning valve and a 10lb residual valve. You have to go out and do several hard stops while driving the car in order to adjust the proportioning valve until the rear wheels no longer lock up. The 10lb residual valve makes a huge difference on pedal feel and how fast the rears actuate.
 
#18 ·
a) You need a new master cylinder. The drum master reservoir is not large enough to supply the front calipers. The most commonly used disc/drum master for manual brakes is the Maverick unit, Raybestos # MC36440. Use with your original pushrod.

b) What is this "splitter" thingy you reference? The brake line for the front circuit (large bowl on the "correct" master cylinder) should go into a "tee" and from there to the left and right front brakes. The brake line for the rear circuit (small bowl) should go into your proportioning valve and then to the rear. A 10# residual valve should be added at the master cylinder port for the rears.

c) Optionally, you can use a "generic" GM-style "combination" valve which will have a proportioning valve, differential pressure valve for warning lamp, and metering valve to delay front brake application all in one unit.
 
#19 ·
I have a set up on my 65. No air in Brake system. It’s not wilwood but reputable front discs brake conversation. With proportional valve. Pedal has a lot of travel. It’s a firm pedal by just too much travel. It’s a dual MC with power booster. I did a good split with valve no lock up. Drums in rear. Someone said a rod that pushes on the MC it is adjustable. Some say DONT MESS WITH IT. INPUT PLEASE
 
#20 · (Edited)
66slvrpny. I used a 1967 Mustang disk/drum master cylinder. I used the power brake version because that's what I have. If you have manual brakes use that version. Autozone is the way to go on master cylinders for whatever reason they are the least expensive. What you referenced was a drum/drum master cylinder that will not work.
 
#21 ·
Great there $50 down the drain due to me not paying attention cause that's the one I bought. I have the manual brakes on mine. So it all brakes down to.... Haha I said brakes... anyway. I need the disc/drum master cylinder with that running to the stock like proportional value and on the line running to the rear drums I need the residual pressure valve? Wow I had this way wrong lol.
 
#29 ·
So yes, unfortunately, you have the incorrect master and it needs to either go or at least have the residual valve in the front removed which is non-trivial. IT is keeping 10 lbs of pressure on the disks which means they are dragging and could be overheating which may be why the are not working. Do they work better on the first stop before they get hot?


I think Woodchuck is right (as usual) you need the larger reservoir. IF you get the correct master cylinder, it "should" have the residual valve built in for the rears. Make sure it does or doesnt as if it doesnt, you need to plumb one in. If you went from a jelly jar to a dual, then the splitter has to go since it takes one input and sends it to both the front and rear circuits. I'm not quite sure how you plumbed a dual into a single splitter but hey...


The ebay valve is fine, as it has the spring loaded proportioning valve that I really like on my car.



Good luck
 
#30 ·
I put the SSBC 4 lug power front disk conversion from Pony Parts into my 66 i6 coupe. The front/rear splitter gets the rear port plugged. The rear left/right splitter is on the rear axle. So now the splitter under the hood only splits left/right fronts. An inline valve came with this kit that goes between the MC and rear line. Oddly, the plug came with the kit but I had to buy an adapter for the front line connection. I wound up the adjustable pedal link about an inch to get a reasonable pedal height. Wouldn't mind it being a bit higher but I didn't want it hanging by a few threads as it were.

Doing road tests, I ended up with the inline valve wide open as that gave me near equal skid marks (too many times watching Dan on Velocity doing this) with still a slight forward bias. Apparently this kit has the MC correctly proportioned already.

I got a small hand held tubing bender from Harbor Freight and it did a perfectly neat job bending the lines. Used some 12AWG solid wire to make a template of the bends and then did the tubing on the workbench. Couple of very minor tweaks and it fit perfectly. I understand that the CuNi is easier to work with but this worked with the short supplied steel lines and a couple small adjustments to the existing lines. Maybe if I get ambitious and redo all the lines I'll go for CuNi. But I have good braking now and it flushed clean pretty quickly, so I don't think the old ones are in bad shape.
 
#33 ·
I think the master cylinder you have listed MAY be a power brake unit (although it says "universal"). Question whether it has the retaining groove for the manual brake pushrod. I have always used the Raybestos MC36440 master for manual disc/drum apps.
 
#39 ·
According to what I saw in the Q and a section of the parts there were quite a few people that said they would work and has work on a 66 Mustang manual brake disks/drum set up. Hopefully this all works so I don't have to dump more money into just being able to have the rear brakes work properly
 
#34 ·
Having read this thread, I now have some questions!

1) What is the function/purpose of the 10# residual valve in the rear brake line?
2) Does the commonly-used '74 Mav disc/drum MC have a residual valve built in?

Just curious. I have the '74 Mav disc/drum MC plumbed with the fronts going to the stock junction block and the rears plumbed through a Wilwood proportioning valve and then straight to the brakes. Can't say I have any complaints but I'm always keeping an eyeball out for something that I missed.
 
#48 ·
Threads that go off in a very educational tangent while still remaining basically on topic are the best! I learned a lot.
 
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#49 ·
From a purely technical standpoint, the simple brake proportioning valve is a "pressure limiting" device. It works by using a spring loaded piston with it's face against incompressible brake fluid, but the opposite side is atmospheric air so movement of the piston (against the spring) causes an increase in fluid VOLUME in the circuit, thus reducing the hydraulic pressure. Adjustable valves vary the spring rate and, thus, the movement of the piston.

Many so-called "proportioning valves" are actually "combination valves" as they consist of 3 individual functions. The first is the proportioning valve, used because during braking weight is transferred from rear to front, thus reducing the force between the tire and the road and unless braking hydraulic pressure is reduced the wheels have a tendency to "lock up" and stop rotating due to brake friction. The second is a differential pressure valve, which senses a difference in pressure between the front and rear braking circuits and moves a "shuttle" piston toward the lower pressure circuit. In some cases it only actuates and electrical switch to illuminate a warning lamp. In other applications it not only actuates the warning lamp but may cover the outlet port to the low pressure circuit preventing the loss of additional fluid and its effect on master cylinder piston actuation. Lastly, there is the metering valve. The metering valve functions by delaying the application of pressure in the front circuit to allow the rear brakes to apply first, keeping the car braking in a straight line.

At least that is what I was taught many many many moons ago when I was young and naive.
 
#57 · (Edited)
From a purely technical standpoint, the simple brake proportioning valve is a "pressure limiting" device. It works by using a spring loaded piston with it's face against incompressible brake fluid, but the opposite side is atmospheric air so movement of the piston (against the spring) causes an increase in fluid VOLUME in the circuit, thus reducing the hydraulic pressure. Adjustable valves vary the spring rate and, thus, the movement of the piston.

Many so-called "proportioning valves" are actually "combination valves" as they consist of 3 individual functions. The first is the proportioning valve, used because during braking weight is transferred from rear to front, thus reducing the force between the tire and the road and unless braking hydraulic pressure is reduced the wheels have a tendency to "lock up" and stop rotating due to brake friction. The second is a differential pressure valve, which senses a difference in pressure between the front and rear braking circuits and moves a "shuttle" piston toward the lower pressure circuit. In some cases it only actuates and electrical switch to illuminate a warning lamp. In other applications it not only actuates the warning lamp but may cover the outlet port to the low pressure circuit preventing the loss of additional fluid and its effect on master cylinder piston actuation. Lastly, there is the metering valve. The metering valve functions by delaying the application of pressure in the front circuit to allow the rear brakes to apply first, keeping the car braking in a straight line.

At least that is what I was taught many many many moons ago when I was young and naive.

This is absolutely true for SOME combo valves but definitely not all and not in our era e.g. 65-70. If I understand correctly, drums front and rear just have a junction with none of those features, 65/66 disk/drum just have the P valve, somewhere 67/68 disk/drum they added the anti-rear-lockup spring e.g. the combo valve from the 86 T bird I am using does not have the metering valve. I believe this to be the case for the valve posted by Nailbender above - so if you want all three functions, you need to source the correct combo valve built with all three installed. I already had my valve plumbed when I learned about the metering valve function. I may try to find one in the future but opening up the brake system is a hassle and it works pretty good as it is.


Thank you bender! That helps a ton! I was kinda in the "I have no idea zone" haha

I'm thinking of this for flaring

GEARWRENCH 41590D Tubing Service Set https://www.amazon.com/dp/B0025Q1WZO/ref=cm_sw_r_cp_apa_i_rTW-CbZPWX2VR

Im a big gear wrench guy
Your local auto parts store has a flaring tool you can "borrow" for free. I use them so rarely its not worth buying.
 
#55 ·

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