1970 GMC 2500 2WD 8 Lug disc brake conversion

[MADMIKE]

I checked the rules, and used the search, but I cannot find anything clear cut regarding the 8 lug(wrong terms? VBullentin 3 letter hiccup?). Trying to find out specifics for what needs to be done. Any and all help will be greatly appreciated. Not sure if I have violated board etiquette with a newbie posting. I will gladly take any internet beat downs if it brings the tech.

I'm assisting my neighbor, yesterday we snagged rotors/calipers/spindles/FUCA+shims/FLCA/springs off a 80s Suburban. But it was apparent we need parts of the steering gear, as his '70 LB outer tierods are small compared to the ones on the Suburban.

Originally Posted by lolife99

DRUM TO DISC CONVERSIONS 67-70

3) The 67-70 a-arms can also be used with any other (71-72 or 73-87) spindle, with a simple ball joint change.
* The upper and lower ball joints and outer tie rods must match the correct year of spindle you choose.

The sleeves and tierod shank diameter on the Suburban are much bigger. We are going to head back this morning to snag the rest of the steering gear.

Outside of 'conversion kits' is there anything else we can do to expedite this conversion? Can he retain his original PS pump/box? From looking at online resources it appears that none of the steering parts interchange. Even the kingpin kits from ACDELCO have different part numbers.

Also what is the preferred or best method for the combination valve?
His '70 has the combo valve mounted next to the master cylinder, the 80s trucks have them mounted to the front frame brace. IIRC the ports are in different spots. Any chance we can just use the guts of an 80s 8 lug P/U combo valve in the '70s? (Probably not).

Hose, I was thinking of just using the Suburban hoses, and if need be, adapters to fit the hard line.

MC, I believe his 70 is 1-1/8". Can this be retained, or do the 80s trucks use a quick take up design?

He would like to keep it relatively simple for parts replacement. If using all of the Suburbans parts are the way to go about it then so be it.

Thanks in advance for any and all help.

[MADMIKE]

Went back and snagged the Suburbans steering gear, we left the PS box and Pitman as it was pretty sloppy. We have the left & right inner-sleeve-outer tierods/centerlink/idlerarm from the 2500 Suburban which is a 04/84 build, if that matters. Looking around at different swaps posted here I am unclear on whether the stock '70 Pitman will work, of if the '84 spec'd unit will need to be used.

BTW is there a faster way to search, waiting a minute to find out that my search net me no results is defeating. Thanks.

[lolife99]

You need a disc/drum master cylinder and prop. valve.
You can use the suburban stuff or buy aftermarket.
The '70 drum brake M/C and distribution block need to be swapped out.
You will have to bend some new steel lines.

I would try the '70 pitman arm.
If it doesn't work,... buy one for the year of the donor truck.

[lolife99]

I like this M/C and prop. valve assembly.
I have this set-up on my '69 swb.
http://www.performanceonline.com/60-...LVE-KIT-MCPVS/

The all aluminum version is nicer,... and won't rust!
http://www.performanceonline.com/HIG...PVS-AMCPVS232/

Or this one, with a booster,...
http://www.performanceonline.com/196...TER-KIT-16667/

[MADMIKE]

Update. Being lazy we just tried installing the inner/outer tierods from the '84 2500 Suburban on the 70' 2500 PU centerlink, and it bolts up no problem. Wheels are at quite a bit of positive camber, need to fix that.

Next fix brake hoses. We're eyeballing '72 disc hoses as they look like they will work. Not too sure on the rear wheel cylinders, at 1-1/8" they are on the large side. Need to get that bias forward. 1-1/4" MC is next.

What have the rest have you done with regards to the MC? The Suburban just has a dimple at the back of the MC, where as the 70 had a recess in the back of over 1.5". Can the shorter rod from the later booster be used or does the whole booster/MC need to be switched out? TIA.

[MADMIKE]

Called up performanceonline and asked about the MCs in their lineup, all of them are 1". That doesn't sit well with me. Using 75mm or 80mm calipers is going to require an MC larger than 1".

So far this is the hydraulic parts list.

'84 GMC 2500 1-1/4" MC which shares the same fitting sizes.
'72 GMC 2500 Booster, appears to mount the same as the '70 but takes newer MC with a dimple on the piston, rather than the earlier 1.64" recess.
'84 GMC 2500 front hoses. A tad long but we'll see.
'84 GMC 2500 80mm front calipers.
'84 GMC 2500 1-1/16" wheel cylinders.
2- 7/16-24 Female to 3/8"-24 Male adapters(rear drum WCs)

This should keep the bias correct, give better stopping power, and shorter stopping distances. Will probably install an adjustable proportion valve later to fine tune the rear brakes if need be.

Kinda bummed that the aftermarket does not offer a correct disc brake upgrade system. Seems like a lot of cobbled together bits that make little sense.

[MADMIKE]

Things to keep in mind.
The '84 Suburban rotors use larger wheel studs, but the stock riveted '70s 16.5" wheels fits. The '70 16.5" wheels clear the '84 rotors/calipers too. No need to replace the rolling stock. Although considering the weight, it would be well worth it for braking, accelerating, and cruising purposes to replace them with lighter wheels.

FWIW, looking up some parts I found that the '68 models had 1" WC with the correct angle for the hard line, but you will still need the 2- 7/16-24 Female to 3/8"-24 Male adapters. These could then be used with the smaller 75mm calipers. Along with the 1-1/8" bore MC of the later trucks.

[MADMIKE]

Originally Posted by k2rm

I have a Performance Online MC running 4 wheel discs on my 1970 C20 and the truck stops really well. This is even running the 77 el dorado rear calipers with parking brake which are known to require a lot of volume to engage.

I am actually strangely familiar with these calipers. 63.5mm pistons if memory serves me right. Every car that used them used a 1-1/8" MC Sevilles/DeVilles/Eldo/Toro/TA/etc.
What bugs me in the Perf Online tech told me that all their MCs use a 1" bore. Now on a non boosted system that's OK with me, but they are using it with their boosted systems as well. I know they use some AL MC, but unless it has a longer than stock stroke, I feel you lose modulation.

Originally Posted by MADMIKE

So far this is the hydraulic parts list.

'84 GMC 2500 1-1/4" MC which shares the same fitting sizes.
'72 GMC 2500 Booster, appears to mount the same as the '70 but takes newer MC with a dimple on the piston, rather than the earlier 1.64" recess.
'84 GMC 2500 front hoses. A tad long but we'll see.
'84 GMC 2500 80mm front calipers.
'84 GMC 2500 1-1/16" wheel cylinders.
2- 7/16-24 Female to 3/8"-24 Male adapters(rear drum WCs)

After a bit of review and back and forth the list was shortened quite a bit.
We had double checked the parts but not everything was adding up. Triple checking showed further differences from the other two sources. I'm thinking the first two had make/model confused.

Comparing parts from GMC to Chevy trucks shows differences. I was a bit surprised that there would be any changes, but there are. Boosters, MC and rebuild kit part numbers are different between the same years. This probably accounts for all the weird discrepancies I was getting while looking up part numbers.

So the NEW list... hah

'70 Chevy Caprice/Impala/Kingswood/etc MC 1-1/8" MC for Disc/Drum cars with 1.63" pushrod depth.*
'70 Cadillac Fleetwood rear WCs 1" with correct angled 7/16-24 ports.
'84 GMC 2500 75mm front calipers.
'84 GMC 2500 front brake hoses.

This will allow bolt in installation with no adapters or changes to the booster.
Although, I will have to check the pushrod depth. '70 2500 calls for 1.64", not sure if the booster push rod is adjustable, or if I will have to shave .01" off the end of the pushrod.

I tried looking up some dimensional differences between the C10 Disc booster(Rod Code CT3/CT5) versus the C20 booster(Rod Code CT4) but only found depth changes by looking at the C20 drum MC vs the C10 disc MC.

*FWIW the Chevy C10 disc brake trucks appear to use the same MC as the Caprice. However, depending on where you look up parts, the GMC and Chevy truck parts get lumped together often with the Chevy specific part being dropped.


And to just add a bit more to this quagmire of parts.
The MC we are planing on using is this unit.


This is apparently is what the Original PB MC should look like.


But this is what the MC that is on the truck looks like.


I suspect that the original owner had the MC replaced at one time but was given the wrong part for whatever reason. I won't know what the bore is until I pull it and report on it. If it is a 1" bore that would explain the long pedal on the truck.

[MADMIKE]

Something I don't think I noted on the FLCA. The bump stop is about an inch taller on the '84 than on the '70. This may not be an issue for anyone who just wants disc brakes on their workhorse, but for those of you looking into lowering your truck the earlier arm will allow for more bump travel. It's not just the rubber that is taller, the metal box that the bump stop attaches to is taller.

[k2rm]

I have a Performance Online MC running 4 wheel discs on my 1970 C20 and the truck stops really well. This is even running the 77 el dorado rear calipers with parking brake which are known to require a lot of volume to engage. A 1" bore should be fine for front discs only. I am running the original 9" dual diaphram booster. If you do get a 1 1/8" bore, make sure you use a large booster like the dual 9". I tried a single 11" first, and braking was very poor. Once I slapped the original dual 9", it stopped really quick.

[lolife99]

Buy the caprice M/C and tell us how it works.
If it was me,... I would buy the 71-72 truck M/C.
They are a different part number for a reason.

[Robznob11]

orielly's offers a cardone rebuilt for the 72 for 17.99+ core. guess what the reman from napa that is 42.00 bucks is also a cardone listed under there part # if you look on ebay you will see the same cardone from 19.99 up to 75.00 bucks all same pt # go figure. I think ill take the orielly's offering at least its a decent brand and it is the one that list for the truck.

[MADMIKE]

Originally Posted by Robznob11

Ok im dealing with this same issue. the master I have on my truck looks like the third on the page and was bolted to a 11" booster. I took it off a donor cab of unknown origin(67-72 chevy or gmc?).

The MC my neigbor pulled off has what appears to be a larger bore than the new MC. I was a little baffled. I have no idea what it is at the moment. I need to dissect this bugger and figure out what the hell was on the truck. Eyeballing it the MC pulled off might be 1-1/4", the bore looks bigger. The new MC is overall, considerably bigger. As my neighbor stated "That's a TRUCK MC there, boy." Comparing the two Centric photos, they are actually that much different in size, 130.64001 is a MONSTER of an MC. I'll definitely have to take pics once we get this sorted.

Originally Posted by Robznob11

I was looking into what master to run ece and cpp both told me that a stock 71/72 disc drum master will bolt right up even though it is 1 1/8 as opposed to 1" of the oe master. now my question is the 71ish capriece master is disc drum and 1 1/8 I like it because it is 26 bucks as opposed to the 45 bucks for the truck one. so what gives is it ok to run the car master.

This was also one of the idiosyncrasies that was pissing me off.
It seemed depending on what time and day of the week it was, I was getting up to five different MCs for the same damn vehicle. It was stupefying. Fact of the matter is there are eight different MCs that were used. Disc or Drum, Manual or Power, and is the MC a Bendix or a Moraine unit.
Bastards.

To simplify your answer, the '70 Caprice PB Disc Moraine MC, and the '72 C10 PB Disc Moraine MC are one in the same. Look up both via RockAuto.com and you will find the same PN, Centric 130.64001.
1" MC is for Manual Brakes
1-1/8" MC is for Power Brakes.
Oh and the Booster under hood has an adjustable rod so I just backed it off .01". Woot.

Originally Posted by Robznob11

also im running a preportion valve from a 85 donor with one front line plugged.

Proportional valves found on production cars are not really proportional. Actually they are combination valves. First it is a junction block for all the lines, with a metering valve for the front brakes, along with a pseudo proportional valve, and finally a shuttle valve to close off a leaking part of the system which also activates the 'brake' light switch due to a system failure.

We are leaving the stock junction/metering block on the truck for now, and will add an additional proportional valve if we find that the rears need less bias.

Reusing a factory combo valve in another vehicle that is not the original application is more than likely not going to be correct at best. Although these trucks share a lot of common or near common parts; weight distribution, balance, and braking performance will differ greatly affecting the accuracy of a pre-programed combo valve. With the adjustable unit we can fine tune the truck for braking when laden or not... if we remember to, HAH!

Ideally if there was some kind of level sensing valve we could install that would be awesome, but I know of no such valve that was used on these trucks.

Just need to get the Fleetwood WCs installed, and get the brakes bled and see how she go's, err stops. But the rain has been coming down, not really the best weather when there's 100% humidity and the fluid you are working with is hygroscopic, bah.

[MADMIKE]

Well I think I should post the wheel cylinders. Keep in mind these are Left/Right specific with the angled port line.

This is the stock rear wheel cylinders on the '70 2500


The stock rear wheel cylinder for 72 C10 with disc brakes.


Its the correct WC size, but this will not bolt up to the stock lines, however...

The stock rear wheel cylinder for a '70 Fleetwood.


The correct rear wheel cylinder size for use with the 75mm calipers, and the correct line port size of 7/16".

[Robznob11]

hey mike just to clarify my prop valve is from a 85 c10 and many people on the board say they run them with no problem. the oe valve if it was a drum set up is just a residual valve and will not work with a disc brake set up.I could just be miss understanding what your saying but any how I think ill be fine with the cardone 72 truck master and the 85 valve. and 17 bucks is cheap enough. I hope you get yours sorted out

[MADMIKE]

FWIW the MC pulled off the truck was indeed a 1-1/8" bore. *shrug* For S&G I called around to local stores to see if they had a MC for a '70 GMC 2500 w/PB, those that did had the screw on cover. So I don't know what the MC was that was underhood. Currently the Centric 130.64001 is bench bled and installed. Awesome.

So my neighbor was tired of looking at disc brake parts and stated he wanted to get to work on the rear drums last night, so we did.
Put the rear on stands, pulled the axles, wheels, then the drum/hubs. A little bass ackwards but meh it was fun and fast. I'm not familiar with Danas, nor floater rears, the diff seems wider than a normal open diff. Kind of hard to see with the spare tire in the way. Well drop that tomorrow for a better view.

Well the Casting brand numbers didn't cross reference correctly, screw em. Seems like members are limited on edits, which keeps folks honest, I do like that. Centric was used for the MC, and Raybestos for pretty much everything else. Admittedly I would have used Centric throughout, but they are hard to find locally(I think the nearest shop that sells to the public is in Vallejo).

Just finished up on the rear drum rebuild. New '68 Fleetwood 1" WCs and the rest was Raybestos replacement parts for the '70 GMC 2500. I dunno why but this was one of the quickest/simplest drum brakes redux done. Maybe it was due to the lack of an axle flange being in the way.

Pro Tip: When doing the drums on these models start with reinstalling the rear shoe. That has the E/P brake attachment and anchor linkage attached to it. Also pop off the C/E clip from teh P/B attachement, don't fight the PB cable on removal/reinstall. The front shoe pretty much installs itself.

The floater rearend is getting a bit of a redux itself. D side oil seal was blown and that rear shoe basically did nothing. The right rear was constantly locking up under hard braking or in the wet. "1/4 turn the steering to the left if you have to get on the binders" should not be told to someone borrowing a truck.
As for floater rear ends. EVERY LIVE AXLE VEHICLE SHOULD BE ONE. I was somewhat dreading the experience but damn it makes everything easier. That C-clip garbage is for the birds.

A few last minute issues that need to be addressed with the front brake lines and the truck should be ready for brake testing.
I'm fairly sure an adjustable prop valve will be added, but we both want to see what it will do before hand.

[MADMIKE]

A couple of things that I don't think I touched on.

Master Cylinder
When converting from drum/drum to discs/drum you have to keep in mind that where on the drum/drum MC the leading port goes to the front and the rear port goes to the rear, on Disc/Drum MCs this is flipped. Front port is for the rear drums and the rear port(closest to the firewall) is for the front discs.

Metering, proportional, and pressure differential valves, oh my!
On my neighbors '70 GMC it does not have a combination valve as I had posted earlier. What it does have is a Pressure Differential Warning Valve. All this is a simple shuttle valve inside, if one of the systems fail the pressurized side will push on the shuttle which will then activate the brake warning light inside the cab. That's it. It does not close off the leaking side, nor is there any kind of metering valve inside.