1972 c20 master cylinder - no match

[Aruba1]

Hi everyone, after an hour seatching through the search function and not being able to get an answer I decided to post a thread :-).

My truck is a 1972 gmc c20 with stock disk brakes. My brake booster went bad so i ordered a brake booster and a master cyl for my truck with disk brakes. The MC has the holes switched, large hole in the front. Should i install this and reroute the lines or should i search for the correct one? Any hint on how to trave the correct part? It almost look like the c10 MC should fit.

Also upgraded the brakes to a 1989 c3500 system (front and rear). Should I get the year correct MC and booster setup for my truck? Thanks

[jocko]

C10 and C20 lines are reversed front/rear on the mc, but then the distrib block comes into play. Reference post 6 of this thread to clarify: http://67-72chevytrucks.com/vboard/s...d.php?t=214559 So, you may have gotten a C10 mc instead of a C20 (pay close attn to which circuit goes to which port on the mc). As long as you bought a no-kidding C20 mc, you should be ok - the fitting size isn't really what matters, it's the line size. I can't really tell in the pic, but they KINDA look the same size (i.e. inner diameter). I don't recommend using a C10 mc and swapping ports, they're designed differently for a reason, max gvwr. Do you have disk rears from the 89?? Using the stock mc for that setup is probably the real answer - but then TBD if the stock mounting solution will work. You want to retain the same pedal to pushrod multiplication ratio.

For disk/drum setup, the front disk lines are usually smaller dia (3/16 if I recall?) than the drum rear lines (1/4"). If all drum, then 1/4" all the way around.

If you go with the 89 hydraulics, all bets are off - I'm not sure which line goes where on that thing.

Keep in mind that if you need a custom booster bracket mounting setup, CaptainFab is your man, he makes em and they are great.

[FirstOwner69]

The one on the bottom in your picture (the new one?) looks like the correct master for a 72 with the original discs.

[Aruba1]

Jocko, the new MC's inner diameter for the front and rear are the same size.

FirstOwner69, correct, the bottom MC is the new one.

So how do I go by getting a correct MC? The one was purchased from Autozone for a 1972 GMC C20 with front disk brakes and it's not the right MC. I guess I can try NAPA and hope they have the right one. And then O'reillys, Advanced, etc. Brhhhh

[Aruba1]

Here is a close-up of the two MCs. The old MC has different inner diameters (small for the front, larger for the rear). The new MC , according to Autozone " a perfect match", has same size inner diameters thus not suitable for a 3/4 ton truck. We have a local NAPA stored that still does the old style match and compare (the owner has been in the parts business for a long long time). I hope he can sort this mess up and sell me the right MC. I just hate when you go buy something that the computer is messed up and the employees are clueless because either a) they have no car knowledge or b) they have no drive to function without a computer or c) they were taught the computer is always right.

[FirstOwner69]

As I said above... The new master does look like those I've seen on 71/72 3/4 ton trucks with front discs. As I understand it, the front chamber is for the rear wheels and the back chamber is for the fronts. 71/72 half ton masters are the reverse. Your old one looks like a 1/2 ton master.
http://67-72chevytrucks.com/vboard/s...d.php?t=628830

[RichardJ]

From "FirstOwner69"'s link with the image below, it sounds like what he is saying is correct.
From the angle of the picture you are showing, your brake likes look home made so I would question the originality of some of your brake components.

[Aruba1]

Yeah, I hear ya'll, they are definitely homemade brake lines but they are routed the same as the 2nd top picture that you posted. And from looking on ebay for C10 master cylinders, its clear that someone installed C10 MC on my truck sometime in the past.

My other question is, on the new C20 MC, is it suppose to have same size inner tubing or it doesn't matter? And are the proportional valves the same for C10 and C20? I am afraid I may have a complete c10 system which is big no no as I am planning on having a lot of weight on my truck.

[Aruba1]

It's confirmed. The old MC and brake booster on my truck was off a C10. The old brake booster center of studs is 3-1/2" and the new one from Autozone is 3-3/4" (correct for C20). Also the new master cylinder has larger rear reservoir (for the front disk brakes) were the old master cylinder had same size reservoirs. And the new MC has 1/8" larger bore. Now I just need to located the right brake lines and the truck will be back on the road. Thank you all for your help!

[Aruba1]

duplicate...

[davepl]

You'll be making your own lines. No one I found has them for the C20/C30 setup. Here are the ones I made. I managed to find the right colored nuts, but used stainless rather than mild steel. Otherwise I think they're close to right.

[mechanicalman]

Originally Posted by davepl

You'll be making your own lines. No one I found has them for the C20/C30 setup. Here are the ones I made. I managed to find the right colored nuts, but used stainless rather than mild steel. Otherwise I think they're close to right.

You found your stickers!

How's your TV cable working?

[swamp rat]

Originally Posted by davepl

You'll be making your own lines. No one I found has them for the C20/C30 setup. Here are the ones I made. I managed to find the right colored nuts, but used stainless rather than mild steel. Otherwise I think they're close to right.

Inline Tube had the lines ( Just like the upper right image in RichardJ's pic ) but it was a kit that includes the lines going to the T fitting on the frame.

[mechanicalman]

Sub'd.

[Aruba1]

i finally got the brakes hooked up and working right again.

[Aruba1]

Pic finally...

[John H.]

I am in the process of having these lines made by The Right Stuff Detailing. They are the first to admit, they need help with some truck lines. The ones they had were incorrect.

[John H.]

[John H.]

[Aruba1]

Maybe others can chime in the part number but I just ordered a booster/master cylinder combo for a 1972 Chevy C20 truck. I am not sure what the PO did to my truck but the new (correct) booster fit right when I removed the worn-out C10 booster. Can you post a picture of your proportional valve? You may need to replace it but I am not sure and bend your own lines from the MC to the valve.

[rx78nt1alex]

I'm pretty sure it is like yours, as it was a disc/drum from the factory, but I will have to wait until I'm 200 miles closer to be sure. I work so I can do more stuff to the truck....

[rx78nt1alex]

So after much research for swapping in a 1972 C-20 3/4 ton brake master cylinder and booster for my 1971 C-10 1/2 ton brake master cylinder and booster, I finally found the sizes and part numbers for making your own lines to connect the master cylinder to the proportioning valve that are crossed as per the factory diagram for C-20/C-30. (Picture 1)

The front most port on a factory booster (part number 50-1007 from Oreilly, with correct large reservoir in rear and small in front) is as follows if pictured by standing on the driver's side fender staring across at the side of the master cylinder (Picture 2)

Picture 3 shows what the part numbers are and the fitting size.

As a side note, NAPA sells an 1/4" x 8" line with the 9/16"-18 blue fitting x 7/16"-24 for about $5.49 each. The green fittings are $3.15 for 5 (5 in one box). I bought a 40" length of 3/16" tubing with other fittings for about $6.70 (I can cut it and flare it with the correct fittings later).

How you bend it is up to you. I'm going to take advantage of the perk of how my employer also owns Ridgid tools and get the benders at discount.