Not the braking I want
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nm9stheham
Well-Known Member
Well all of that sounds good and it looks like it should. Nice installation, BTW!
Did you get air from both MC chambers when bench bleeding? You should have. And was this done on the bench or in the truck? The concern with the rod adjustment is that the pistons are able to slide all the way back to the stop when the brakes are released. Was the rod adjustment into the MC done by measuring? Since you can't see it when assembled, that is the only reliable way to do it that I can think of. If it is too long, then the pistons won't go all the way back.
BTW, if you press on the brakes really hard with the truck not running, do you get a hard pedal with the pedal not too far down?
Did you get air from both MC chambers when bench bleeding? You should have. And was this done on the bench or in the truck? The concern with the rod adjustment is that the pistons are able to slide all the way back to the stop when the brakes are released. Was the rod adjustment into the MC done by measuring? Since you can't see it when assembled, that is the only reliable way to do it that I can think of. If it is too long, then the pistons won't go all the way back.
BTW, if you press on the brakes really hard with the truck not running, do you get a hard pedal with the pedal not too far down?
nm9stheham
Well-Known Member
BTW, do you know WHICH GM calipers that you have or the piston diameter? If you got the smaller one, which have 2.38" diameter pistons, then that is smaller than the Mopar ones, and will reduce front brake torque for a given pedal pressure. They will put about 18% less pressure on the pads than the stock A body Mopar calipers with their 2.6" pistons.
Knowing the rear brake drum diameter and shoe width would help too to get some idea if the front pistons are too small.
Knowing the rear brake drum diameter and shoe width would help too to get some idea if the front pistons are too small.
I Did the bench bleed on the bench. I did get a lot of air, it took awhile.
I adjusted the rod a bit long and test fit the master, I slowly backed it off until the master freely bolted tight to the booster.
Pedal seems to go down a ways and get's hard.
When I called on the brake stuff he said they are GM metric calipers (78-85 Comaro or Monti Carlo)
nm9stheham
Well-Known Member
OK, well the 'metric' calipers have the smaller piston size. So that may be what is going on: with a given line pressure, there is less braking torque on the front than on the rear. This is reason that the rear brake size has been asked.... to see if there is a mismatch.
The rear brake torque has to do with the drum diameter, the shoe width, and the bore size of the rear wheel cylinders and the shoe materia. The front brake torque has to do with the rotor diameter, the caliper piston size and number, and the pad material. Without good data on all of these, it is kinda hard to call. So any more data you can provide is of help.
Try cleaning the front pads and rotors again just to make sure it is not something silly. I'd pull the front pads to see if they have been contaminated with anything.
If it is just too small a front brake torque the options generally are:
- put in calipers with bigger pistons (earlier GM ones)
- put in smaller rear wheel cylinders if they are available
- change material front and/or rear (Probably easiest on the fronts, like EBC Yellowstuff compound)
- put in a rear prop valve..... but it almost sounds like the F to R mismatch may be great enough to not just depend on that; I'd rather figure up where the bias generally is right now, and get it close with changes and then use a prop valve to fine tune it in
BTW MBM lists their PN PV2C as a GM prop valve but it may just be a brake failure switch and distribution block.
BTW#2 do the rears have manual or auto adjusters inside?
The rear brake torque has to do with the drum diameter, the shoe width, and the bore size of the rear wheel cylinders and the shoe materia. The front brake torque has to do with the rotor diameter, the caliper piston size and number, and the pad material. Without good data on all of these, it is kinda hard to call. So any more data you can provide is of help.
Try cleaning the front pads and rotors again just to make sure it is not something silly. I'd pull the front pads to see if they have been contaminated with anything.
If it is just too small a front brake torque the options generally are:
- put in calipers with bigger pistons (earlier GM ones)
- put in smaller rear wheel cylinders if they are available
- change material front and/or rear (Probably easiest on the fronts, like EBC Yellowstuff compound)
- put in a rear prop valve..... but it almost sounds like the F to R mismatch may be great enough to not just depend on that; I'd rather figure up where the bias generally is right now, and get it close with changes and then use a prop valve to fine tune it in
BTW MBM lists their PN PV2C as a GM prop valve but it may just be a brake failure switch and distribution block.
BTW#2 do the rears have manual or auto adjusters inside?
Of all of those suggestions, only using calipers with larger pistons or changing the pads will improve the braking on the front. Changing the rear wheel cylinders will reduce the rear braking, but it will do nothing for the front. Same for the prop valve. It will reduce the pressure to the rear brakes, but that doesn't increase the pressure to the front.
Brakes are very sensitive to the length of the pushrod, a small discrepancy there can make a big difference in the pedal travel and pressure generated. And even the small piston Metric calipers should be more than enough to hold the truck. The rear brakes locking up before the fronts is normal in a system without a prop valve (or a custom system where you need an adjustment), the rear shoes are held a very short distance from the drums and the way they're applied causes the shoes to grab faster, which is one of the reasons disk/drum cars come with prop valves to begin with. Plus with it being a truck and light in the tail end you'll need a lot of front/rear bias to keep from locking the rears up first anyway. But, like I said, that doesn't account for the front disks being inadequate, just for the rears locking up.
Do you guy's know what calipers I would need to upgrade too for better braking up front? I did order an adjustable inline prop valve for limiting the back brakes some.
"BTW MBM lists their PN PV2C as a GM prop valve but it may just be a brake failure switch and distribution block" Not sure what this means?
"BTW MBM lists their PN PV2C as a GM prop valve but it may just be a brake failure switch and distribution block" Not sure what this means?
nm9stheham
Well-Known Member
OK, these are standard Gm pads' I assume for the metric calipers with the 2.36' pistons. The FF indicates a Coefficient of friction around .35 or .4; these are likely semi-metallics.
EBC Yellowstuff CoF is around .5 so just putting that pad material in will increase front brake torque by around 25-30%. But I would rather put in large pistons to start, and then tune in the pad material. Here is one link to 2.75" bore caliper that I think will fit exactly.... I'll leave it to you to be sure.
Big Bore GM Metric 1978-88 Brake Caliper
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nm9stheham
Well-Known Member
It means I don't know exactly all the parts that are inside that so-called prop valve.... LOL It DOES have a brake failure switch, that goes off if there is a leak on one half of the system. But, I don't know if it really have a rear prop valve; the 'thing' on the back where the rear line connects may be it but that looks odd for a prop valve; the 'thing' looks more like a residual pressure valve.....but I really don't know on this one.
BTW, I asked about the rear brake adjusters..... if they are manual adjusters, and you have them tight, then the rear will tend to grab sooner than the fronts. I used to change (mess with...LOL) the initial brake bias on my Opel rally cars by messing with the manual adjusters.
nm9stheham
Well-Known Member
Well, what we don't know here is the size of the rear drums. Dana 60 truck axles can have some pretty big rear brakes. It could very easily be far more rear brake than the fronts. It would be good to go over any setup or installation problems you can think of.. it sounds like the OP feels he has things bled and cleaned up right but we aren't there to check directly. That always makes internet advice just that: advice.
But it doesn't matter how big the rear brakes are. If the fronts aren't holding, there's nothing that an adjustable prop valve or smaller wheel cylinders in the back will do to improve the braking in the front. Yes, doing either or both of those things will change the front to rear brake bias, but it will not strengthen the front brakes, only reduce the braking force of the rear.
nm9stheham
Well-Known Member
Well, let's go over the symptoms described:
This indicates that the fronts just are weak with the pedal pressure modulated so that it will still allow the rears to turn due to engine torque overcoming braking torque. The general solution to this is more front brake torque relative to the rears (i.e., a bias change) or a line lock which the OP has suggested.
But then he goes on to say that the rears lock up in normal use:
So there is clearly an underlying bias problem that can be solved by reducing rear brake force (with smaller rear wheel cylinders or a prop valve) or by increasing front brake torque.... which we have been discussing.
This all makes sense because 11" front discs and 12" or 13" rear drums that are 2" or more wide is VERY likely waaaay off balance. This is likely the issue: The rear brakes are so strong that the OP has to let up pedal pressure to get the rear wheels to spin, but at that lower pedal pressure, the front brake torque is too low to hold. If things are way off balance, my preference is to better equalize the F & R brake torques before fine tuning with a prop valve.
Now, if there is something still wrong in the fronts that are causing them to not have as much brake torque as they should, then the OP needs more suggestions one what to to check it out. I have gone through a fair list, and he seems to feel like it's good. But it is sure possible that some other item has been missed.
BTW, the pix of the pair of front pads look like the are clean to me.
This indicates that the fronts just are weak with the pedal pressure modulated so that it will still allow the rears to turn due to engine torque overcoming braking torque. The general solution to this is more front brake torque relative to the rears (i.e., a bias change) or a line lock which the OP has suggested.
But then he goes on to say that the rears lock up in normal use:
So there is clearly an underlying bias problem that can be solved by reducing rear brake force (with smaller rear wheel cylinders or a prop valve) or by increasing front brake torque.... which we have been discussing.
This all makes sense because 11" front discs and 12" or 13" rear drums that are 2" or more wide is VERY likely waaaay off balance. This is likely the issue: The rear brakes are so strong that the OP has to let up pedal pressure to get the rear wheels to spin, but at that lower pedal pressure, the front brake torque is too low to hold. If things are way off balance, my preference is to better equalize the F & R brake torques before fine tuning with a prop valve.
Now, if there is something still wrong in the fronts that are causing them to not have as much brake torque as they should, then the OP needs more suggestions one what to to check it out. I have gone through a fair list, and he seems to feel like it's good. But it is sure possible that some other item has been missed.
BTW, the pix of the pair of front pads look like the are clean to me.
This all makes sense because 11" front discs and 12" or 13" rear drums that are 2" or more wide is VERY likely waaaay off balance. This is likely the issue: The rear brakes are so strong that the OP has to let up pedal pressure to get the rear wheels to spin, but at that lower pedal pressure, the front brake torque is too low to hold. If things are way off balance, my preference is to better equalize the F & R brake torques before fine tuning with a prop valve.
I think the above hits the nail on the head exactly!! You sir are a wizard hahahahaha
Im going to order the caliper kit you linked me too today. Also I think im going to order slotted rotors for the front as well if you think that will help? I need 11" 77-82 Ford Grenada rotors
Would I be better of with a willwood caliper? Wilwood 120-8924 D154 GM Metric Iron Single Piston Floater Caliper
I think the above hits the nail on the head exactly!! You sir are a wizard hahahahaha
Im going to order the caliper kit you linked me too today. Also I think im going to order slotted rotors for the front as well if you think that will help? I need 11" 77-82 Ford Grenada rotors
Would I be better of with a willwood caliper? Wilwood 120-8924 D154 GM Metric Iron Single Piston Floater Caliper
nm9stheham
Well-Known Member
Well, slotted rotors don't increase brake power; the slots are there to shunt off gases that boil off the pad surfaces when they get really hot, and can actually form a layer of gas that gets between the pads and rotor surfaces. That helps in really hard use, like road racing. Buuuut, they usually have a bad effect on pad wear: I have seen one case where they chewed the pads off in a matter of 15-20 racing miles! So for normal use, I would not think to go with slotted rotors.
The Wilwoods you linked look to be the same caliper. I can't think thye will be any different, unless there is a quality difference, but I can't answer that. The key is to get the larger pistons. This will go in the right direction, IMHO. But without complete dimensions on the rear shoes, drums, and wheel cylinders, it is hard to know if it is enough. So at this point, you are just shooting at a solution.
BTW, a lot of what I learned, I learned from others. So if it is useful, the credit goes to a long line of racers and hot rodders and tinkerers and engineers.
The Wilwoods you linked look to be the same caliper. I can't think thye will be any different, unless there is a quality difference, but I can't answer that. The key is to get the larger pistons. This will go in the right direction, IMHO. But without complete dimensions on the rear shoes, drums, and wheel cylinders, it is hard to know if it is enough. So at this point, you are just shooting at a solution.
BTW, a lot of what I learned, I learned from others. So if it is useful, the credit goes to a long line of racers and hot rodders and tinkerers and engineers.
nm9stheham
Well-Known Member
And I'll just insert the standard disclaimer..... internet advice is worth every penny of its price ..... LOL
I sincerely hope that we are on the right track; it's the best I can do with he data provided.
I sincerely hope that we are on the right track; it's the best I can do with he data provided.
Maybe. I run 13" front rotors and 11"x2.5" rear drums with the stock '74 disk/drum prop valve on my Duster. There's no crazy imbalance. Nor was there with 10.98 front disks, or 11.75" front disks, as I've run all them on that car. The rears lock up slightly before the fronts in panic braking, but that's true of ALL of these cars from the factory, at least every one I've ever driven. If he's just got a distribution block from a drum/drum application (like that truck was originally) then I could totally see that being the issue. But if there's any kind of prop valve on the truck it shouldn't be as far off as what's been described.
But it's the internet, who knows. Maybe the new parts will fix it.
nm9stheham
Well-Known Member
Of course not.....you have more in front than in back. But the OP has it reversed with more bake than normal in the back vs. the front sizing. If you need a prop valve with your setup, with the fronts enlarged more than normal, and still lock up the rears slightly, imagine what 11" rotors and even bigger rear brakes will do to further push the bias to the rear.
It's the best I can make of it with limited info. If you have other thoughts on what is happening, and fixes, then it would be good to lay them out.
Well, the OP made the change hoping for better,but was he experiencing this problem before, or did the swap create &/or
exacerbate the problem? FYI,mopar used 3.10" piston calipers
on all disc brake combos,8.25 thru dana60 w/duo servo 12X3"
rear drums.Using 11.75" or 12.82" rotors as the change up frt.
If your new MC has a larger bore/piston for the frt discs,you've
lost braking power,I've seen that happen before. It looks as tho'
you have the proper metering valve at the frt. lines(under the
rubber cap),but it's impossible to tell what pressure it's set at.
Mopars OE cut-in was 117psi, if yours is too high,you will get a
higher amt. of rear apply B4 the frts. start working. The valve in
the rear may be the residual check valve,which holds 5-10lbs. of
pressure against the wheel cyl. seals to keep them from drawing
in moisture/air when they cool down. Some MC's have this in-
corporated in them however. Lastly, you could see if you can get
a smaller wheel cyl. dia. than what you have now to balance things.
exacerbate the problem? FYI,mopar used 3.10" piston calipers
on all disc brake combos,8.25 thru dana60 w/duo servo 12X3"
rear drums.Using 11.75" or 12.82" rotors as the change up frt.
If your new MC has a larger bore/piston for the frt discs,you've
lost braking power,I've seen that happen before. It looks as tho'
you have the proper metering valve at the frt. lines(under the
rubber cap),but it's impossible to tell what pressure it's set at.
Mopars OE cut-in was 117psi, if yours is too high,you will get a
higher amt. of rear apply B4 the frts. start working. The valve in
the rear may be the residual check valve,which holds 5-10lbs. of
pressure against the wheel cyl. seals to keep them from drawing
in moisture/air when they cool down. Some MC's have this in-
corporated in them however. Lastly, you could see if you can get
a smaller wheel cyl. dia. than what you have now to balance things.
nm9stheham
Well-Known Member
He put on a whole new front end with Mustang II parts, 11" rotors and the metric GM caliper, with pistons of 2-3/8" diameter. So it is a whole new setup. Prop valve is a GM.... I have no idea at what pressure it cuts in.
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