The CNC 702 3/4" Master Cylinder

These are a well made part featuring a stainless steel insert. Not light like the Tilton or Wilwood's but everything is metal and thick. These are used on the Mustang's brake set up. These often find their way on to dune buggies and sand rails. Highly recommend them.

Removing the Master Cylinder Piston

Using the snap ring pliers remove the snap ring. Makes sense... The piston should pop out with the retaining washer.

Close Look at the Piston

This looked very good. After looking for any wear marks or dirt I was beginning to thing that the used master that was in the set was only used for testing or something as it does not look like it has seen any service other then a fill of fluid or possibly they used the wrong sized master and swapped it out. In any case looked good and was happy to not see any marks in the stainless bore either. Once the piston is out you likely will have a rubber piece stuck in the bore.

CNC-702 Lid Removed

With the lid and rubber gasket removed this is what it looked like. Pretty clean except for a bit of dried up fluid.

Removing the Rubber Cap

I used a bit of air to blow it out. I think you could use a non-sharp dowel or something thin to get it. Air seemed the best way to go.

Piston Seal Cap or Whatever...

Not sure what you call this part, but it's on the end of the piston. Don't worry a new one comes with the rebuild kit if you botched it up.

Piston Assembly

Here is the piston assembly and how it goes together. You can also see the large opening washer that is held in by the snap ring in the upper right.

Push Rod Side with Snap Ring

This is the push rod side of the master. You can see that the snap ring that must be removed. It's SPRING LOADED so be careful in case it pops out.

Wrong Rebuild Kit

Thinking I had one of these in the tool box, I thought I got lucky. Looked perfect except for one small detail... This was for a slave cylinder and the piston has a different depth center bore then the master cylinder. Oh well. This one was the 907R which is not correct for the master.

Subtle Piston Differences

The pistons that came out of the rebuild kit I had (the 907R) was for a slave cylinder. It has a short bore vs. the one that came out of the master cylinder. So while darn close, not going to work. I ended up waiting for the proper kit even that the removed piston was virtually new.

Inspect the Bore

The bore in this master was clean and pretty much unused. If yours has bad scratches it might be worthy of a replacement rather then a rebuild. I guess you could hone it but I'm not sure worth it. Would be a good idea to disassemble things before ordering a rebuild kit as a new master might be the way to go.

Clean the Master Cylinder Body

After things are apart time to clean the casing or body (What ever you want to call it). I used some brake cleaner and blew out all the holes with air after cleaning. Be wary that the small bleed hole in the picture is clear and clean. The 2 larger holes closer to the edge are easy to clean as well. Air helps blow anything out. Try not to use any rags that may have lint or other left overs.

Insert Spring Assembly Into Body

Insert the clean spring assembly into the master cylinder body as shown. More part are coming!

Install the Cap or Plunger or what ever it's called!

Carefully install the rubber cap (plunger??) into the bore. I used a very light coat of PRB Rubber Lube on it just enough to help with fitting it. Since the rubber is larger then the bore you may have to use something to CAREFULLY get it in, again CAREFULLY is the key word. This sits on top of the aluminum end that's inserted into the spring.

Insert the Piston

Insert the piston with the O-Ring facing up. The flat (with no opening) should be facing the flat of the rubber seal. Again a very light coat of PRB rubber grease was used on the O-Ring to help slip things in.

Slipping In the Piston

Ok, should not be too hard. Again a touch of PRB Brake Grease on it (I guess you could use a dab of brake fluid too). Piston should be oriented so the O-Ring is up or you are screwing up the install.

Install Snap Ring and Retainer

On the home stretch now. Just need to install the retaining washer and the snap ring to lock the assembly in. I used a large phillips head screwdriver with some paper towel stuffed into the bore of the piston to keep it from marring up (Don't forget to remove it!!!). You could also use the brake push rod if needed. Main thing is that you will need to compress the piston into the body enough to allow you to drop in the retaining washer and get the snap ring into it's groove. Using some nice angled Knipex pliers were helpful.

And Done!

After the snap ring is released into the groove you are about done. While CNC mentions not to bench blead the master it still may be a good thing to do, not sure why they don't want you to.

As with any project involving brakes it's a good idea to test things before going out for a run. So when replacing the master you should follow common procedures to blead and test your brakes, if you don't you may die!

Conclusion

This is a pretty easy project. If you master is in good shape upon disassembly order the rebuild kit and have at it. Double check part numbers and piston size before clicking the order button just to be sure you are getting what you need. Otherwise common sense, cleaning, and easy assembly are all part of this DIY rebuild project.

Happy and SAFE Motoring!