I need some brake help.

[bax]

37'' tires, 1/2 ton chevy disk on the front and 3/4 ton chevy disk on the rear. I have an astro van hydro boost with a corvette master cylinder. The master cylinder is a disk / disk design. I am currently running without a proportioning valve in the lines. All my lines are the same Dia. 3/16''. Problem is I am not getting much braking from the rear and the whole system is a bit squishy. No matter how much I bleed the brakes it is pretty much the same, Shitty. So. What did I do wrong? Willing to change about anything except the brakes themselves. Should the rear line be bigger Dia.? do I need a different master? a disk disk proportioning valve? I know brakes are overrated but I want a little more.
Any Ideas????

 

[gr8scott]

How did you run the lines from the master? The vette masters work a little differently than the eb masters. The front reservoir go to the
front brakes, and the rear to the rear. Also, do you know the bore size of your master? I found 1" and smaller resulted in very squishy
super sensitive brakes. 1 1/8" works perfect. If all the above is in order, then maybe some 2 lb residual valve(s) might help.

 

[bax]

It's a 1 1/8'' bore. Master on the bronco is higher than any other brake component. The valves you speak of are for retaining line pressure when a master is mounted below the brakes. It might work but it's not it's intended use for a bronco.

 

[gr8scott]

Yeah true, I was just throwing ideas out there. My rear drum brakes didn't work all that great until I put a 10 Lb residual valve in the line,
I know it doesn't make sense but it worked for me. So are you lines run front/front, rear/rear?

 

[bax]

Oh I get it and it might work. yes the front goes to the front and the rear to the rear. But I think the corvette master is equal. You could hook it up backwards and it would still work. Both front and rear main lines run to a split on the diff's. Then to the calipers.

 

[gr8scott]

You could hook it up backwards and it would still work.

Yeah I tried it like that at first. The brakes worked but felt really weird. Hard to describe. Did some research and found out
that in the vettes of that era the master was mounted "backwards" relative to our EB's. So mounted backwards theirs were
front/rear, rear/front. Don't think you need a prop valve running disks all around. Others will chime in with ideas I'm sure.

 

[Mike Bernard]

I have the wilwood 1 1/8 master cylinder which is a copy of the vette mc and willwood prop valve. I had to adjust the prop valve all the way to the back brakes to get them to work right.

 

[bax]

Good to know.

 

[bmc69]

Had to step up to a 1.25" bore MC to get the mushy feel and long pedal out of the hboost setup on my '78.

 

[bronconut73]

I still have manual 4 wheel disc but I really like the feel of the bigger bore MC. Yes it is a stiffer pedal but I feel like I can better modulate the pedal if the brakes lock up.

I tried the 1" stock, the 1 1/8" common replacement, then finally the '73 or '74 F-250 Camper Special 1 1/4" MC.

The biggest bore MC firmed up the pedal and cured many other issues too.
Still have it but may go down in bore size when going hyrdoboost since our community has that process dialed in. Won't try to re-invent the wheel when doing hyrdoboost.
Having said that going to the big bore MC was a game changer for me, and a direct bolt-on to boot. Oh and this MC was cheap as shit too. Like $19 bucks at AutoZone 12 years ago,

 

[bax]

I am good going to a different master. 73/74 f250 C.S. I can try that.

 

[bax]

I guess I need to look for an 1.25 Dia master cylinder that is set up for disk disk. I wonder is a 2000 f 250 would work?

 

[bronconut73]

Mine is not for disc/disc.
I just made sure it had no residual check valve and rocked on.
Under extreme hard breaking the back brakes begin to lock up but you can easily modulate that with your foot.
Almost perfect for manual brakes.

I have always sort of figured that the increased volume of the bigger bore made up for any thing lacking in the MC from its disc/drum heritage,

 

[DirtDonk]

It's possible that the rear calipers are just too big to start with. You said 3/4t brakes on the rear? Are those rear brakes, or fronts that have been re-purposed? If fronts, they're probably just too big.
In this case a larger master may just help, but not sure if it will get rid of it.

I always think a prop valve in the rear line (manually adjustable type) is a good thing to keep in mind no matter what. But even more-so with larger brakes on the back.
The proportioning really doesn't have anything to do with the master size or reservoir location, or whether it's for disc/disc or disc/drum originally. At least not when both the front and rear brakes have been swapped out to something non-stock to begin with.
The adjustable valve is just in case the rears are too strong.

I don't have one on my '71, but even though I planned for it, never ended up needing it.
Every rig is different, especially when non-stock.
But if the rear calipers are too big to begin with, maybe that's something to keep in mind if the larger master does not help.

Wasn't it bmc that just related that issue and that the smaller rear caliper helped the issue?

Paul

 

[Cortez]

Bax - any issues with bleeding? It’s been a few years but when I did my hydro boost I remember spending several days trying to get a good bench bleed on the the h-boost unit and was able to finally get it done... I remember learning some new vocabulary- it was a bear. Also did an gravity bleed process on the rest of the system which helped, no matter how much I worked on bleeding it with speed bleeders I was never right until I tried the gravity bleed process.

 

[Apogee]

As for what you did wrong, best guess is that you're running front calipers in the front and rear of a short wheelbase, high CoG rig.

That said, I have a few questions:

1. What is the piston diameter of the front calipers? Or year/make/model you bought them for? The assumption is a 2.94" piston diameter with 6.79 in^2 effective piston area, but there are large piston variants of those calipers that are otherwise identical with respect to mounting, brake pads (FMSI D52), etc.

2. Same question for the rear calipers...what are they?

3. Front rotors are presumably Ford Ø11.72" x 1.20", so what rear rotors are you using?

If I assume that you're running 1987 or older 3/4-ton GM front calipers in the rear, as opposed to parking brake calipers, then most likely the biggest issue you're facing is that the master cylinder bore size is insufficient for the given caliper piston area. Front calipers up front, no biggie, but front calipers front and rear can be an issue depending on caliper piston areas. While stepping up to a larger bore master cylinder should fix your pedal issues (nvrstuk had similar problems on his most recent build updates), it won't do anything to address the front/rear bias problem.

You can add an adjustable proportioning valve in order to try and reduce the rear brake bias slightly per what Paul said, however those are not meant to overcome a major component mismatch if that is indeed what you have and would not do anything to address your pedal issues. Instead, you could swap out the front calipers in the rear for the parking brake caliper equivalent as offered on the 76-77 Cadillac applications, which have smaller piston areas (Ø2.03") and would reduce the rear bias accordingly, as well as the fluid volume requirements, making the 1.125" bore MC more suitable for the application.

If you choose to stick with the calipers you have, like you indicated, then I'd suggest that you run a bigger bore MC and deal with the premature rear brake lockup and over-steer condition it creates. Your best bet at that point might be a front/rear friction stagger to reduce the rear brake torque to the point that it's at least safe to drive in a panic stop situation. Higher CoF up front, lower in the back...see where you land. Adding an LBS type of valve could help band-aid the situation further.

As for line size, that has nothing to do with anything really. The MC determines how much fluid is supplied to the front and rear brakes based on the bore size since we assume brake fluid to be incompressible, and the line size just carries the fluid. The only thing line size determines is fluid velocity, and to a certain degree, system compliance since larger lines expand more under pressure than smaller ones.

Last but not least, you could convert over to a dual master cylinder setup where you could stagger the MC bore sizes to match the calipers, since the relationship between the master and slaves (calipers) is important.

HTH,
Tobin

 

[bax]

Chevy calipers front and rear. Front are the typical kit / pieces used in a bronco application. The rear are a C30 3/4 ton chevy truck with a front ford bronco rotor. Corvette master with a 1 1/8'' bore , disk disk. I do have a rear adjustable portioning valve but the rear brakes don't grab much at all. Nothing too fancy just not working very well. I think a larger master may be the first thing I try.

 

[bronconut73]

https://www.jegs.com/i/Allstar-Performance/049/ALL41064/10002/-1

I have this one but it's the cheap oem version of the rebuilt MC from Auto Zone. 10 years strong.

With the big bore you need strong calves, lol.

 

[bax]

Just the one I was thinking about. I looked at it last night. Thanks for the link, now I dont have to remember what I looked at.

 

[Apogee]

Chevy calipers front and rear. Front are the typical kit / pieces used in a bronco application. The rear are a C30 3/4 ton chevy truck with a front ford bronco rotor. Corvette master with a 1 1/8'' bore , disk disk. I do have a rear adjustable portioning valve but the rear brakes don't grab much at all. Nothing too fancy just not working very well. I think a larger master may be the first thing I try.

In general, I would want a 67/33 to 75/25 front/rear brake bias on a rig like a lifted EB due to the short wheelbase and high CoG, as this results in a lot of forward weight transfer under braking, thus reducing the amount of rear brake you can effectively utilize under heavy braking.

So if you have the typical 1/2-ton Chevy front calipers with Ø2.94" pistons and 6.7 in^2 piston area, and the rears have something between Ø2.95" and Ø3.38" pistons (depending on the year C30 and RPO code JB7 or JB8). So this means that you could have something close to a 50/50 front/rear brake bias all the way up to a 33/67 split depending solely on the rear caliper piston diameter and respective effective area. I'm assuming the same front and rear brake pad CoF, which I think you're going to want to change to a friction stagger if the goal is to be able to achieve threshold braking without premature lockup of the rear brakes. If your rear calipers are larger than the fronts, then I would suggest swapping them front to back and back to front if you're not willing to change anything else, as this would at least lessen the front/rear bias imbalance.

Regardless of application, brakes should be designed to stop the vehicle in the shortest distance for which the tires and chassis are capable in order to maximize the safety of the driver and those sharing the road with you. If I were to spec a preferred rear caliper piston diameter (assuming single piston) for your application, I would choose a 1.68" to 2.05" [Ø42.5mm to Ø52mm] caliper piston diameter...something like the Ford Explorer kit (Ø48mm).

While still not ideal, TSM does offer a Ø2.4" piston parking brake caliper that is interchangeable with the C30 front caliper for $320/pair that would at least get you closer, and might be within the range of what an adjustable proportioning valve can handle with respect to tuning.

https://www.tsmmfg.net/new-not-rebuilt-large-gm-rear-calipers-3120.html