1959 Cadillac Brakes Issue

[Jeff Wilk]

Why is anyone going through all the effort of modifying the firewall?

I just finished my conversion to a dual MC but i kept the original booster and had it rebuilt by Midwest

Midwest then supplied a new dual MC that bolted right on. Once we realized he sent the wrong one (one without the residual check valve internal to the MC) he sent the correct one.

That bolted right on also and know i have a dual MC system with the rebuilt original booster without having to modify the firewall. Only modification at all is rerouting of a new line to feed the front brakes from the master to the junction block. A plug in one port of the junction block for where the rear line used to go. And an extension SS line from the stock rear line that used to go to the junction block so that it now connects to the new dual MC directly.

This new (correct) dual MC also clears the cruise control unit with ease unlike the wrong dual MC i had in there.

[Gabe Davis]

Why is anyone going through all the effort of modifying the firewall?

Probably being cheap. The rebuilt '68 booster and master cylinder is about $100 bucks. So for that, a few minutes with a file, and some washers and fittings, you can get a dual MC setup. My booster was totally trashed so this ended up being the best (cheapest) option for me.

[Bill Caddyshack]

Once we realized he sent the wrong one (one without the residual check valve internal to the MC) he sent the correct one.

What is the "residual check valve internal to the MC"?

BTW, I probably would have done what you suggested if I had thought about it. 

[Jeff Wilk]

I am NO brake expert and just learned about this valve. If you look in the shop manual you will see it shown as an internal and integrated part of the MC. For drum brakes it holds from 2-10 lbs psi (depending on the valve) in the system to give the booster basically a head start. The large brake return springs on each wheel/drum hold the shoes back from the drums when no pedal is pressed.

Disc brakes dont need these and some new MCs dont come with them internal and you need to use in line valves in those cases where needed. My original factory single MC had one like they all did. The first dual MC i got did not. As a result we could never "get a pedal". Once the new MC with the internal residual check valve came as it should have originally all was fine.

Here is a final picture.  Factory original and rebuilt booster with correct type of MC simply converted to a dual.      Only mistake.......i forgot to paint this second MC as i was so anxious to finally get this two year brake job back on the road!!!  For those who did not see my son and my original postings,  what started as a blown wheel cylinder backing out of garage in spring 2011....became a full front clip and front end restoration due to a case of ......"while we're at it..... That firewall looks real bad now that the booster is off.........then wow while we are at it.....now that ac/heater box looks real bad since we did the booster.......then wow, now that the ac box and booster are done those wheel wells look awful......and now it is 2013!!!!"   BE CAREFULL out there 

[Coupe Deville]

This question is for Bill Caddyshack. Since I have the same booster and M/C as you. Did you need one of these check valves? Is you brake pedal good? Besides the modifying of the brake lines and firewall did you have to add anything to your brakes? Thank you

[Bill Caddyshack]

This question is for Bill Caddyshack. Since I have the same booster and M/C as you. Did you need one of these check valves? Is you brake pedal good? Besides the modifying of the brake lines and firewall did you have to add anything to your brakes? Thank you

I am trying to understand what Jeff means by "residual check valve", and whether I have one or not?

[Jeff Wilk]

Check shop manual for detailed description.

[INTMD8]

Well I think I may have to just send my original master/booster in for a rebuild. Summit sent me a letter saying the dual mc is on eternal backorder and I can't find one anywhere else.

[tzurick]

Since I plan on installing disc's up front I ordered the combo unit for front disc and rear drums for the 68 Deville.  We'll see if it ships or not.  Currently it has an ETA for middle of September.

AAZ-50-1121 -> Brake Booster and Master Cylinder -> Expected Arrival on 9/17/2013

[tzurick]

Seems like you should be able to use the 1121 on all drums if you just add the residual check valve in for the disc side of the master cyclinder.  Any way to tell if you have this check valve built in or not?

[Bill Caddyshack]

It appears that a residual check valve is a good idea for drum brakes and a bad idea for disks. The valve keeps a constant line pressure on the brake shoes which is good for drums, but bad for disks.

Here is some info that seems important. I also include a link to the full article.

"There is a simple though not foolproof method of checking to see if your system has residual pressure. With the brake system at rest, open a bleed screw on a wheel cylinder or caliper. If even a few drops of fluid come out, residual pressure is probably present. If the wheels are off the ground on a disc brake car, depress the brake pedal firmly and lift off. Attempt to turn the wheel just slightly to feel the amount of drag. Open the caliper bleed screw and close. If the wheel now turns more freely, residual pressure is probably present.

Occasionally, another problem in the brake system will be mistaken for a residual valve. This is when the master cylinder pushrod length has been improperly adjusted in such a way, as to not allow the piston to return all the way to the back of the bore. If the piston does not return all the way, the cup in front of the piston will not move back far enough to open the bleed hole. When the bleed hole is not open, residual line pressure is the result. Unfortunately, this type of residual pressure is not controllable and gets worse the harder the brakes are applied. Master cylinder pushrods always should have "free play" to insure that the piston can return all the way back when pressure is released."

http://www.thebrakeman.com/valvetechi

Since the 50-1117 Cardone 1967-68 MC is for the all-drum models, it has residual check valves. But maybe not? I'll check mine using the above, but I wonder -- does anyone here know -- if it would be good to have an additional residual check valve in the brake lines? One for front and one for rear.

This could be added at the MC?   

[Bill Caddyshack]

More info. Just got a response from Cardone who re-manufactures the 50-1117 Booster/MC. This unit does NOT have a residual check valve.

The RCV prevents brake fluid from backing up in the MC

A lot of people are saying that you don't need an RCV if your MC is higher than the wheel (calipers,cylinders?). Some cars have the MC lower. Maybe this applies more to disc brakes (?)

But, it seems a good idea to me to have RCVs. The mechanical logic is that the springs that pull back the brake shoes in the drum brakes exert a back pressure on the brake fluid lines that forces fluid back into the MC. Then when you step on the pedal, it has to travel more for the shoes to contact the drums.

The RCV maintains about 10 psi in the brake lines. This is not enough to have a drag on your shoes, but enough to keep them in closer proximity to the drums. This gives a higher pedal.

I am researching RCVs that will fit our 1/4" lines. Will be back with more results.

[tzurick]

I was reading that drums should actually have closer to 10 PSI RCV and 2 PSI for DISC's.  The 2 PSI for disc's in supposedly achieved just by effects of gravity having the master cyclinder mounter higher than the calipers.  You should just be able to look in the master cyclinder output port and determine if the RCV exists.

Bill -> Ask them about the 50-1121 if you have a chance.

[Bill Caddyshack]

I was reading that drums should actually have closer to 10 PSI RCV and 2 PSI for DISC's.  The 2 PSI for disc's in supposedly achieved just by effects of gravity having the master cyclinder mounter higher than the calipers.  You should just be able to look in the master cyclinder output port and determine if the RCV exists.

Bill -> Ask them about the 50-1121 if you have a chance.

Further research shows this to be correct. The 10psi (Red color) RCVs are correct for drum brakes. Summit Racing has them. I may mount them inline. "Brakeman" suggests mounting them on the MC, but bright red and 2.5 inches long? Not so pretty in an older Cad. Guess I could paint 'em blue.

http://www.summitracing.com/parts/wil-260-3279

I will be checking locally tomorrow to see what is available.

[tzurick]

Since I'm going to be doing all new lines and disc's up front I'm leaning towards installing the standard GM Proportioning Valve when I do it.  Everything that I read says it's dual purpose in that it proportions and maintains residual pressure.

As a bonus you could hook up the warning switch to the brake light on the dash.

[Coupe Deville]

Do I need a check valve or one of these things to just have brakes? Won't the brakes work without it?

[Bill Caddyshack]

Since I'm going to be doing all new lines and disc's up front I'm leaning towards installing the standard GM Proportioning Valve when I do it.  Everything that I read says it's dual purpose in that it proportions and maintains residual pressure.

As a bonus you could hook up the warning switch to the brake light on the dash.

Sure, why not? It would serve a dual purpose. I have mixed emotions about the disk brakes. Do they really make a big difference in stopping power in normal driving?

I suggest that you buy the parts from a reliable source that did not have them made in a junk shop in China. I also wonder if the disks get the white walls dirtier. Seems like I read that somewhere.

[Coupe Deville]

So i need a residual check valve and do i need a Proportioning Valve? I have all stock 4 wheel drums. When i got into this brake project all i thought i had to do was add the new lines for the rear and make the firewall holes narrower. It seems to be getting complicated now.

[tzurick]

The proportion valve is not needed for all drums.  If you were to install discs up front and change nothing then in a hard stop your rears brakes would lock up first without the proportion valve and you wouldn't like that very much.   The residual pressure check valves you want if the master cyclinder doesn't have it built it.  Your stock master cyclinder had it built it.  The new one you have sounds like it doesn't so you would want to add an inline one to each line.  Will they work without the valves...sure....your pedal with just travel farther before you actually feel the brakes engaging.  You master cyclinder is mounted higher than your wheel cyclinders so you won't have the drain back issue.  These extra parts in a sense optimize the system and make it perform better.  The pressure valves are cheap and easy enough to do.

[Coupe Deville]

Would it be possible to get a new master cylinder that has the valves built in? A new master would be cheaper then two RCV and i would not have to see them when there installed inline with the brake lines.