Technical Brake help before crash and burn

Hello.
I'm a bit cornfuzzled and hoping there's someone out there that can lend some help. Normally two heads are better than one and maybe someone can see it differently.

A while back I added an "upgrade" to my 52 Chevy styleline by installing a set of disk brakes up front and a master/booster combo under the floor off a fabricated bracket welded to the crossmember. I had read thru a bunch of old H.A.M.B posts about what guys were doing and I formulated my own approach. The mounting and brackets are not where I'm having the problem. BTW thanks H.A.M.B. for all the old posts and search option!!

All new 3/16 brake lines front to back, new 1 3/16" master, small 7" booster, all new rubber lines front and back. New rear 1" wheel cylinders. New large GM (not metric) calipers, pads, and rotors. 2 LBS RPV to the front (because the master is under the floor), and 10 LBS RPV built into the proportioning valve to the rear. After driving the car for a while I noticed the front dove on deceleration and would sit still on declines with the clutch pressed in, so back to the H.A.M.B. I went and read what people had to say. A lot of posts had said that RPVs aren't needed in some cases and to remove them. I removed the 2 LBS RPV and the brakes unlocked. Now that the RPV has been removed I can't get volume to the rear. I'm able to bleed the system but it trickles out like an 80 year old man with prostate issues.

Am I supposed to be running 1/4" line to the back to increase volume to the 1" wheel cylinders so the system will see proper pressure?

And help would be greatly appreciated!!
I miss driving the car.

Thank you
Jesse.

 

Just a question, are you using a dual/tandem master cylinder?

Pat

 

Yes. Reproduction GM master.


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Not sure why is sent copies.


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Copies happen to some, system hiccup..
Back to brakes: Are you sure the Proportioning valve has a 10 lb RV? Do a pic..

 

Hydraulic pressure doesnt care what size your lines are. 1 3/16 master is a large bore. Is this comparable to a factory system? By that I mean, I might look at a 1960 F100 for master and wheel cylinders sizes so I know what fits what

Brake problems are so common, that brakes need their own sub-forum

 

Seb. It's a CPP proportioning valve. I called them and verified it was built in. I also tried giving the brakes a quick double pump to build pressure, doesn't work!


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I understand that pressure doesn't care about line size! But don't wheel cylinders require volume as well as pressure?!


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thats why I asked, Are your master and wheel cylinders sizes part of factory sizes? I look at existing combinations to learn what works. In other words is there a car that uses the sizes you are using?

 

Yes.
The guy I buy my parts from has been in business for longer than I've been alive so he knows I'm going to pick his brain when I come in. The wheel cylinders cross reference into disk/drum vehicles into the 70s. That's why I don't understand where my issue is coming from?!


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A 1 3/16" master should move a lot of fluid, but if you lock the fronts in the first say, 1" of pedal travel, nothing gets to the rears. I've had similar problems with "designed" kits for tri 5 chevys and other popular cars. I don't trust any combination valves anymore. If I'm starting from scratch I use separate rpv's and an adjustable valve on the rear.

You could try bleeding the rears with the fronts open, and see what you get then with a full pedal stroke. I finally broke down and bought a set of gauges so I could see what kind of pressure I had in each circuit. It also lets you see the 10lb residual if they read that low, or you can sub in a regular pressure gauge for low presssure readings.

If you get to 80 before that happens, you're doing better than most.

 

Miker. Thank you! I'll try your suggestion tomorrow. I didn't mention that the rear end is from a 1955 Chevy. Other than paint there's nothing I have not touched in this car. The 235 is out of a 61, it's 30 over, Patrick's cam, Langdon 2X2s, buffalo T5 adaptor, new tank, new wiring, new radiator, driveshaft, geeeesh everything has been touched, re-worked, and fixed. That's why this kills me!


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What product do you prefer using?

 

Wilwood. Not cheap, but cheap brakes aren't in my program. I'm sure there are others that are fine.

http://www.wilwood.com/MasterCylinders/MasterCylinderValves.aspx

 

Thank you!
I appreciate everyone's help!

 

Do you have your lines reversed at the master cylinder?

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Any time you put on disc brake you need to have a proportioning valve so the fronts do not lock up before the rears.
I would just get a proportioning valve that you can adjust manually.

 

^^^^^^^^^^^^^^What He Said!^^^^^^^^^^^^^^^^

 

I am absolutely using a proportioning valve. That is not the issue.

 

You asked for help, so I will throw this in..My old chevy was giving me fits!!!! It would take on air in the lines, but did not leak fluid! result was after a drive the brake "boom" to the floor. pull over to the side of the road wife slide over in the seat and I would open the bleeder back on the road! I have 4 piston calipers from ~ 75 corvette and after 2 years of searching I found a guy in florida (corvette stainless st. brakes) that told me how to fix the problem called "rotor runout" If your set up DID work, you don't need to reengineer

 

What was his solution? What fixed the runout?

 

You have that completely backwards. The fronts should always lock up before the rears, and that's what rear proportioning is designed to accomplish.

 

Like I said before in my original post, I'm not having an issue with locking up at all. Low volume is my issue. When I originally purchased this "set up" I made calls and asked questions. I was told by a major manufacturer that all this would work together harmoniously.


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Why does almost every brake problem I see here involve a Corvette type master? I think a 1" bore might be better for your car. On my Ford the master is a Mustang disc-drum type. I like to be able to go back and know what the origins of my parts are especially with brakes. I would be using that with your car, but does the booster you have play well with it? I went manual with my Ford and use a CPP proportioning valve. So far it is wide open and the car stops well, no rear lock-up.

 

Are you sure that master cylinder is for disc/drum combination? Usually, when both fluid chambers are the same size, the MC is either disc/disc or drum/drum. A disc/drum MC has two different size chambers, with the larger one connected to the disc brakes. This is because the disc caliper pistons are so much larger than the drum wheel cylinders and need higher volume than the drums. The internal workings of the MC (valving, location of ports, position of seals, etc) are specially designed to work with the different requirements of the disc and drum brakes.

 

Is your master bore 13/16" or 1 3/16"?
With both ports open and no vacuum ***ist, make sure the master can completely bottom out before the pedal runs out of travel.
Make sure the pedal can return farther than the booster allows, and that the pedal has it's own return spring.
The 2 lb. residual in the front prevents any fluid drain back, and may be necessary in your case.
Check with the booster supplier on the distance between the output rod and master cylinder primary piston. OE specs are usually about 1/32", and this is measured with full vacuum (about 20" Hg) applied to the booster.

 

Good morning!
Here's where I'm at currently.
I've taken a step back from trusting the major manufacturers advise.
Last night I did an evenings worth of research sitting face to face with a parts guy and going over bore sizes, and line sizes! I've devised a plan of attack and I'll keep this thread going until I get this all sorted out.

Bobss, I've come to the conclusion that the reason everything says 'CORVETTE' is because the general public thinks that's the best. Top of the line.
I restore and preserve early 60s to late 70s GM vehicles and I know that to be untrue. The 72 Vette i have on my rack currently is a m***ive turd.


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Thanks bob.
Ironically went thru those steps a couple nights ago. I get enough over the counter parts to know that 'specks' vary after they have been touched my the re-manufacturer. What I'm dealing with is like the piston does not return to seated position allowing for the valving to replenish fluid.

 

Originally had 2LBS RPV installed in the system and they were causing the fronts to drag and lock up. I removed them but kept a coupler where they use to be in case I eventually re installed.