I have a 1973 Corvette with a little over 32,000 miles. Most of the components are original to the car so when the master cylinder failed I decided to rebuild to keep everything original. The issue is according to the NCRS manual the current master cylinder is not original to the car. I have tried to look up information on the number that I have found and the there are something that do not add up. Please look at the attached pictures and let me know what you think.

My car is equipped non-power brakes, but if the numbers are 5460346 the master cylinder is for a power braking system

 

That does have the part number for a PB system.
And the stamped date looks like the 298th dat of 1968.

There is a machined surface in front of the cylinder facing up. Does it have a "PG" stamped into it? That would also be for a PB car.

I think the non PB cars part number ends in 509. At least it does for 69 cars but I think its the same for 73.

 

The part # on my early 68 manual brake MC ends in 509. In my AIM it shows 3 different part numbers for manual, power and heavy duty power brakes. Seems odd that a PB MC would be on a manual brake car. I guess anything can happen on a 40+ year old Corvette.

 

Thanks, I was looking at the manufactured date and thought that it was 3298 which didn't add up if the car was build in Feb 73. Maybe the car had power brakes from the factor and I should be looking for a power booster, I guess the only way to tell is to look at the build sheet.

 

It could be 298 of 1973. The number stamped looks like CN82983. So it could be anything. I didn't see the last 3 at first. Its a mystery. But knowing if this is an originally equipped PB car would help with solving the question. But as you stated, the date still wouldn't line up.

 

Thanks for the help, I have decided to rebuild the master cylinder and install a power boost unit to the brake system. I will post reply and let everyone know how it turned out.

 

I have the Power Brake booster that came off my 73 I'm not sure if its original but can look for markings if you are interested. I went with the chrome replacement setup on my Restomod.

Tim

 

The master cylinder was cast Feb 6 1973 per the cast date tag.

 

I thought cast dates on master cylinders are Julian dated. I see 263 but I don't think the 2 is Feb, 6 is 6th day or the last 3 is 73. I was under the impression it would stand for 263th day of whatever production year they made it which may not be known. Same as the back of a smog unit. Those are 3 digit Julian dated parts which leave out the year also and can not truly be verified by which year it was made. Its assumed that the date on the aluminum part its attached to would indicate the year the Julian date represents. I assumed this is the same with the MS. For example I have a air pump stamped 0169 which is Jan 16, 1969 and the rear cover cast date is 353 which is assumed to be the 353 day of 1968.

When the cast date is like engine blocks B 6 73 it would be Feb 6 1973.

If anyone can clarify master cylinder cast dating I would be interested in hearing about it. Not much is out there. Even the date stamping on the machined surface is not consistent. I've read everything I could find on it and sometimes the 4 digit Julian date can have the year first then the 3 digit day or it can have the year last with the 3 digit day first all in the same production year.

Very confusing indeed.

 

If you clean off firewall below master cylinder if it was factory PB would have hand written crayon " Mark" PB

 

While the conversion (back?) is certainly doable, there is more to it than adding a booster and the vacuum tube and hose that connects to the carb.

The hole that goes through the firewall is different. Might be a good clue as to what it originally was if you unbolt the M/C and post a pic of the hole behind.

The pushrod from the brake pedal to the back of the booster is different than that for a manual brake one that goes directly to the M/C. It is connected to a different hole on the brake pedal too.

Last item is that the striker plate for the brake light on the pedal is also different. The striker plate and the clevis for the pushrod switch positions in the brake pedal. The idea is that you need more leverage on the M/C for manual brakes. That's also the reason the M/Cs have a smaller bore size for manual brakes.

 

If you really want power brakes, rebuild the M/C, install the booster, and do the other stuff necessary to get P/B operational.

If you just want to repair the M/C, buy a CORRECT replacement for the non-power M/C and install it. Done deal.

It is most likely that the M/C in the car is a replacement that was improperly selected. BTW, having power brakes will not make your car stop any better of improve brake fade problems; it will only reduce the level of effort YOU exert to actuate the brakes.

 

I purchased a SS sleeved m/c for my '69 non p/b car years ago. The casting # was for a P/B car. I think it was a power m/c sleeved to use a 1" piston for manual brakes. You might want to check the piston diameter.

 

Cylinder cases cast at the Flint foundry only used the last # of the year. IE 2 12 3 is Feb 12 1963 1973 1983. Most castings used this format. Some SB Tonawanda cases did have the last 2 #'s of the year.

 

Interesting!

 

If your referring to engine cylinder cases then I think your format is wrong, at least by the mid 60s, in that they used a letter to represent the month. FEb 12th 1968 would be represented as B 12 8. At some point in the 70s they used two numbers for the year. Before 1965 I don't know how they did it. They did this format with water pumps, rear end covers, rear end cases, exhaust manifolds, strut supports and maybe some other parts

Since we are talking about the master cylinder above, the cast 263 or 283 is a Julian date, its not month day year. Its the 263rd day of some year. Its the same as what you find on the back of the smog pumps, a 3 digit Julian date cast number. They also hand stamped the master cylinder on the machined surface with a 4 digit Julian date that showed the year either on the first position or forth position. On smog pumps they also hand stamped a date on a machined surface but the format was more consistent.

There is also a hand stamped date under the proportioning block for the brakes.

 

Just my $.02, as I recall, back in the day P/B masters were used to convert manual master cylinders to SS bringing them down to 1" for manual brake cars

 

You are correct. I must have been off my meds or had my head up my ***. Most casting dates are cast on using letters for the month #'s for the day & a single last # of the year as you said. I hate putting out misinformation.

 

I ordered the rebuild parts for my MC thinking it was a power unit and found that the bore was to small. I order a new no power MC to get the car on the road and plan to take the component of it to rebuild the original since the parts was available for the manual MC. Thanks for the assistance.

 

From looking at your pictures, I don't think you have the correct master cylinder. 1973 Corvette master cylinders had bleeders on them, above where the brake line fittings attach. The bleeders were eliminated during 1974.

The 1 1/8" bore, "346" MC, was used on all 68-82 Corvettes with power brakes, and 67's with J-56. The 346 casting was also used on 67-68 power brake Camaros, early power disc/drum 69 Camaros, as well as assorted Firebirds, Chevelles, Tempest/GTO's, Cutlass', etc.

There are 4 variations of the 346 casting used on Corvettes. 67-72 346's have bleeders and a half moon shaped pad machined in the front of the casting. A 2 letter identification code was stamped onto the machined pad. 73 and first design 74 346's have bleeders, but no pad or ID code. Later 74-76 346's have no bleeders, and a deep hole piston, while 77-82 use the same casting with a shallow piston. All of them have a 1 /8" bore. Power brakes became standard in 77, and the booster design was changed, which is why the piston type changed.

All 67's (except J-56), and manual brake 68-76 Corvettes, used a "509" casting. There are 3 variations to the 509, but all have a 1" bore.

I'm wonder if you have a GM service replacement MC. This is just a guess, but since they were no longer using 509 castings for production, maybe GM used the 346 casting for all service replacement parts, and just machined them differently depending on the application?