Brake master cylinder replacement

[mopar440]

    Someone put a FF- 80558 brake mastercylinder on my 71 chally ,its  getting very soft, weak, so i tried to order a replacement  only to be told the ff-80558  wasnt a correct replacement for that year to start with. Hmmmmm... i have drum brakes a 318 . the ff-80338 im replacing has a larger resovior for the front brakes. I might up grade to discs later. the correct mastercylinder my parts man ordered, and says is the right one, is equel  size resoviors{10-1493m onbox} and 29887 stamped on cylinder.  MY QUESTON IS THIS , IS IT SAFE TO REPLACE THE BRAKE MASTERCYLINDER WITH A LIKE REPLACEMENT WITH THE THOUGHT OF UP GRADING DOWN THE ROAD OR SHOULD I STICK TO CORRECT EQUAL RESOVIORS FOR DRUM BRAKES . basicly i dont want to replace or upgrade to a disc brake mastercylinder if i can saftly use one now with drum brakes. Ps any help  appreciated 
including proper part number for a usable safe cylinder to use THANKS!!!!

[the_engineers]

I'd be concerned about mis-matching brake system parts...it is what keeps you from hurling into things.  What are we talking, $20-50?

Buy the correct one now.  Buy the appropriate one for discs later.

[Changin Gears]

The reason disc brake master cylinders have a larger resovior is that the calipers do not adjust as the pads wear like drum brakes.  All the adjustment is just the piston moving out and staying out of the caliper. It take more fluid to do this, so you get a larger resovoir to hold it.

[DoNotChallengeHer]

Stick to equal reservoirs. With the large and smaller ones. It will blow your wheel cylinders. Don't ask me how I know 

[alphabuck]

i just went through this on my 71.  my car was built in november of 1970.  the original master cylinder on it is what every last aftermarket brake company from raybestos to a1 cardone to wagner would call a 1970 model.  it has one bolt in the middle that holds the cap on.  the resources i have ( i work at a parts store ) all show the 1971 models using the master cylinder that the lid is held down by spring clips. your 10-1493m is the spring top design.  id be asking for a 10-1323m or if youre not worried about it being exactly OEM get the brand new one 13-1323m.  35 bucks over the counter for all new stuff.