Master Cylinder Replacement

[Doppleganger]

Probably a stupid question, but I'm past due for another one anyways, so here goes....

Looking for new replacements for my booster/master cylinder. Found an NOS original booster, but now am looking for the master cylinder.

My original one -

You must be registered for see images attach

I know the reservoir looks full....its actually bone dry as this vat of burnt coffee looking swill was actually my brake fluid -

You must be registered for see images attach

Rock, etc all say that for 13" rear brakes I need this style:

You must be registered for see images attach

So....other than the style of reservoir, does it matter?

Thanks.

 

[Nonstop]

What size are the rear brakes and what is the brake code? I do not believe there should be a problem, but they differentiate between 11 and 13 inch brakes.

 

[fast68chevy]

that is wrong one and i highly doubt there is 13" brakes on that 80s K20. because 13" brakes had the iron m/c that you are looking at, and your truck does not.. you most likely have JB6 brakes.

youre doing something wrong. you need that aluminum 80s style m/c like you have now. dont try to put something different on it. just gonna get yourself into trouble.

you need the exact right one. they have different piston bores and possibly different line ports size and threads also.


the JB or JD brake code on your RPO sticker label will tell you what brakes system you have so you can get the right m/c
JB5 or JB6 or JD6 or JB7 RPO code.

JB7 and JD7 are 10.5 axle and 13 brakes, iron m/c
the others are 9.5 axle which is 11 brakes and alum m/c
your truck has a JB5 JB6 JD6 m/c and thats what you need.



you can measure across your brake drum if you get under the truck, you wont evne need to measure accurately and be able to tell quickly whether it is 11 or 13 across a drum..

do you have the 9.5" SF axle or the 10.5 FF axle?
if its 9.5 i can already tell you you def dont have 13"
there were also some 11" brake 10.5 axles too, lighter GVWR trucks. under 7500lb. and no HD chassis equipment RPO


you should go by the RPO JB code.


and replacement aluminum m/c used to not come with a reservoir you had to clean your original one out and swap it over to the new m/c

 

[Doppleganger]

I can find nothing with the JB codes on my SRO sticker (and its in pretty nice shape). The truck is a heavy 3/4 ton and does have the 13" rear brakes. All the drum hardware and shoes are 13's.

My reservoir looks like the illustrated 11" reservoirs (and it could have been swapped at some time in 35 yrs....PO's son was a cerebral-plegic that screwed with everything else). Dunno. I would think the reservoir style is semi-irrelevant.

 

[Turbo4whl]

You must be registered for see images attach

One advantage with this master cylinder, it is supplied with the vent window unlocking tool.

 

[Craig 85]

Do you have one of these stickers on your driver's door still (photo #1). It may answer some questions. I've owned numerous Squares since 1989 and until this year, I didn't know there was a light duty K20 available in the 80's. This could help explain what brakes your truck came with. If the tag is there and 215 tires are spec'd, then it was a light duty K20. I've owned 4 1985-1988 C/K20's with 13" rear brakes and I only recall an all metal M/C. Maybe the plastic M/C was for light duty K20?

The second photo is from my 1985 sale brochure that shows the Lt duty C/K20's.

You must be registered for see images attach

You must be registered for see images attach

 

[Doppleganger]

Do you have one of these stickers on your driver's door still (photo #1). It may answer some questions. I've owned numerous Squares since 1989 and until this year, I didn't know there was a light duty K20 available in the 80's. This could help explain what brakes your truck came with. If the tag is there and 215 tires are spec'd, then it was a light duty K20. I've owned 4 1985-1988 C/K20's with 13" rear brakes and I only recall an all metal M/C. Maybe the plastic M/C was for light duty K20?

The second photo is from my 1985 sale brochure that shows the Lt duty C/K20's.

You must be registered for see images attach

You must be registered for see images attach

That whole area where the tire size is shown almost looks like melted fingerprints on my sticker (which is almost faded blank to boot) - totally illegible. But the GVW reads 8600 lbs. and my rear drums are no doubt, 13".

I know the column was removed and replaced because the guy's kid couldn't replace a broken ignition switch.....it was so fubar'd that you couldn't put the truck in gear with the tilt wheel up or the shift lever would hit-jam against the steering wheel, plus 3/4 of all the harnesses under the hood - and to the column under the dash were cut off / gone. The shifter linkage to the trans is also a mess....makes me wonder why thats all cobbled too. So I wouldn't be surprised if the master cylinder, booster or both were swapped too...and it would have been a used part swap like everything else.

I have the truck to a shell - next up is pulling the cab and bed off. Just want to get this thing back to where it should be. Does it matter which master cylinder I put in it (especially if its the HD one)? What would the differences be between a light duty and a heavy duty (besides GVW)?

 

[Craig 85]

The bores of the all cast unit is bigger than the plastic one. It would effect the volume of fluid it could push.

 

[Doppleganger]

AC Delco has a master cylinder 'with plastic reservoir' for 11" brakes. The AC Delco Professional for 13" brakes, is the cast one. These are definitely 13" brakes so I think I'm going with the cast.

I have taken (no joke) about 800 pics of this truck so far (for reassembly purposes) and noticed there was a strap with a harness that was hanging next to one of the nuts on the MC, and the nuts dont match, so I think its been off / maybe swapped?

EDIT: Here's what I've found. Front calipers for trucks with 11" or 13" rears have different PNs. JB6 is the 11" rears, plastic reservoir and a front caliper piston of 2-15/16" for vehicles up to 7200 lbs. JB7 is the 13" rears, cast reservoir and a front caliper piston of 3-5/32". I have found nothing as far as a different bracket, so am wondering if you had the JB6, could you upgrade to the JB7 (on fronts at least). I have 13" rears as well as the 3-5/32" piston'd front calipers, so the JB7 brakes except for that smaller / wrong MC, so going to replace it with the one with a larger cast reservoir.

 

[Raider L]

Turbo4whl,
What is a "vent window unlocking tool"? You are referring to the brake res. right?

 

[ali_c20]

Uuuh the brake master cylinder can of worms was opened

Here is some info on brake GM codes, maybe someone can need it.

JB1 : BRAKE, MANUAL, DISC/DRUM, 4300 LBS
JB3 : BRAKE, POWER, DISC/DRUM, 4600 LBS
JB5 : BRAKE, POWER, DISC/DRUM, 6400 LBS
JB6 : BRAKE, POWER, DISC, DRUM 7200 LBS
JB7 : BRAKE, POWER, DISC, DRUM 8400 LB5
JB8 : 8RAKE, POWER, DISC, DRUM 10000 LBS
JC4 : BRAKE, POWER, DISC, DRUM 5600 LBS
JC5 : BRAKE, VAC POWER, 4-WHEEL DISC, 7200 LBS
JC7 : BRAKE, POWER, DISC/DRUM, 12000 LBS
JCB : BRAKE, POWER, DISC/DISC, 20000 LBS
JD3 : BRAKE, HYD POWER, DISC/DRUM
JD5 : BRAKE, DUAL POWER, DISC/DRUM, 6400 LBS
JD6 : BRAKE, HYD POWER, DISC/DRUM, 7200 LBS
JD7 : BRAKE, HYD POWER, DISC/DRUM, 8400 LBS

 

[AuroraGirl]

Turbo4whl,
What is a "vent window unlocking tool"? You are referring to the brake res. right?

i think the retaining clip can be pulled off and used as a strong wire to stick in and open the window, thus allowing acess to your locked truck. fooled if you have plastic reservoir or inside hood release

there were two styles of iron mc reservoirs i think, power that is.

 

[AuroraGirl]

You must be registered for see images attach

You must be registered for see images attach

this is what i meant

 

[custodian]

Probably a stupid question, but I'm past due for another one anyways, so here goes....

Looking for new replacements for my booster/master cylinder. Found an NOS original booster, but now am looking for the master cylinder.

My original one -

You must be registered for see images attach

I know the reservoir looks full....its actually bone dry as this vat of burnt coffee looking swill was actually my brake fluid -

You must be registered for see images attach

Rock, etc all say that for 13" rear brakes I need this style:

You must be registered for see images attach

So....other than the style of reservoir, does it matter?

Thanks.

I have a rebuilt like the first picture for JB5.

 

[Swearbody]

Am I mistaken for thinking you can measure the thickness of the rotor and determine if you have the 13s or 11s
In a previous build of mine I ran across me having the 1.25" rotors instead of the 1". The truck was optioned with the HD brakes per build sheet and had an iron master reservoir. So in that regards you could measure the thickness and determine which size....
Again I could be way off but thats what I ran across.