Massive master cylinder confusion...

[diesel4me]

I decided after counting no less than 20 empty pint bottles of brake fluid in my 82 GMC's bed today,that it was time to replace the master cylinder--for about a year now its been slowly dripping out of the rear seal,and the rear chamber will run dry if you didn't top it off before every drive--I could go 1-2 weeks without it going empty...(so,my reluctance to get another master cylinder cost me about 50 bucks worth of fluid!..)..the brakes worked fine as long as I kept it full,so rather than have to replace it in the dead of winter,I kept adding fluid--it's July 15th now..)..

The master cylinder I had on the truck,was one I saved off a 77 GMC K2500 I ended up junking ,I had bought it at Autozone just a few months before I scrapped the truck--I guess it didn't like the long nap in my shed,right after I installed it after sitting about a year,it oozed fluid out of the back of it almost immediately.....

When I went to order another master cylinder last saturday,the computer records at Autozone showed I had bought one for another truck I had long ago,a 1979 C-10 2wd...but--they had no record of me buying one for the 77 GMC K2500 ,...and if I still had the sales slip ,it would not be readable anyway...the counterman said "that one for the '79 C-10 was a lifetime warranty,so we can just swap it for the defective one"...sounded good to me...

I had looked up the number online for the master cylinder previously,and showed it to the guy,he said "it looks different than the one on your truck now"--I explained how that one now on it had two large equal sized chambers,but the "right" one, original to the truck, had one with two chambers of different sizes (which I still had for a core)...the only difference was I had to use adapter fittings to get the lines to fit it--which I have seen come in the box sometimes with a rebuilt master cylinder..

He said he could not order me the one my truck had originally,he could only swap the defective one ,for one identical to it..

I know it would fit and work,so I said go ahead and order it..went and got it today...and it looks like the original one did,with two different sized chambers....

Funny thing happened when I went to pay for a lower hose they also ordered at the same time..the girl rings up my slip,then hands me 6 bucks and change,and says "hmmm--" I told her " I think I owe YOU ten bucks + tax for the hose--the master cylinder was a even swap for a defective one"...

She called the manager over,he looks over the slip,and says "no,thats correct--the price for the master cylinder went down from 23.99 when you bought it (8 years ago?)--to 13.99 now!...so,I ended up going home with the hose,the master cylinder --AND 6 bucks and change !....I still think they screwed up--oh well.....not every day I go home with free parts AND get money back!..

I put the master cylinder on,bled it,and the brake pedal feels firm,I have not yet driven it but I'm sure it'll stop OK...the lines fit with no adapters needed,and I was glad they came off easily without twisting the tubing up..

The only thing bugging me now is I dont know what GVW or brake system my truck had--I think it should be a JD6 (7200 lb GVW) or JD7 (8400 lb GVW) code,having 13" rear brakes--the GVW sticker is fubared on the door and its not listed anywhere else..

The master cylinder I got today was a M1521 and looking online I see thats listed for 1/2 tons with or without hydroboost with JD5 brake systems,which would be 6200 lbs..

I did notice the bore was a bit smaller than the one I replaced,by maybe 1/8" ..not sure if that will be an issue or not..
I think thats the only difference between the M1521 I ended up with,vs the M1372 the listings called for on my truck looking it up online..

I'm sure it'll still stop OK,the same part number shows up on a lot of other years and models and also many years of GM full sized cars like Caddy's and Buicks...I rarely carry a lot of weight and being a diesel,speed isn't usually an issue either..just hope this one dont LEAK!..

I see the master cylinder on my '85 diesel Burb also has an empty rusty rear chamber now too...and the one on my '81 van that sat ten years ,I could barely push the pedal down last time I tried the brakes--I pushed harder,and the rear line popped..the inside of the master is pure rust now.....sitting KILLS trucks!..

 

[evolve991]

Yep sitting kills trucks and construction workers too. I've been noticing a wet brake booster lately myself. It was an 85 diesel but I swapped a 350 in when it threw up diesel and water all over the driveway. I was told,assured and ridiculed, that I HAD to swap out the hydroboost for vacuum...before I found this forum....and somehow that involved a plastic master cylinder. I'll be slapping a metal one on it. I know for sure it's a JD5 system now let's see what kind of circus the "parts people" can make of this.

 

[84400]

When I went to disc brakes I swapped in a master for a 1977 c3 vette. The vette had disc/disc and it worked better then my stock one. Also had to swap out the p valve. .

I was surprised to see the vette master uses same size fluid cambers.

Its nice when the auto store gives you cash. Had that happen last year for a similar warentee.

 

[84400]

The worst thing for a vehical is sitting. especially the brake and fuel systems.

Mercedes Benz maintance suggest a full brake system flush every year. might seem over kill but my fathers 1984 300CD with 350,000 miles is still running all original brake parts aside from the pads/rotors.

 

[diesel4me]

I've seen trucks that get used daily go 25+ years with no leaks in the brakes hydraulics..and I've had ones that sat 6 months and needed new calipers and wheel cylinders..since brake fluid absorbs water,I'm sure it gathers up enough to rust out the cast iron in brake cylinders in no time..

No one ever bothers to flush the brake fluid--though it should be done regularly..up here,rust usually drains ALL your brake fluid from a blown brake line every 2-3 years,unless you use the nickel-copper brake tubing..

Been doing some snooping online and I found the specs for the M1521 master vs the M1372 are the same--both 1-1/8" bore,and I suspect the M1372 might be in the process of being phased out,and the M1521 being substituted..it's not uncommon for rebuilders to consolidate part numbers..

I dont see a M1372 listed in A-1 Cardone's online I.D. guide,(but its still listed for sale on many sites!)--and many of the masters I looked up say "may differ in appearance from original,but is designed to fit and work as OEM" also..so I guess I'm picking nits here--probably cant tell the difference from the seat of my pants between the "original" master cylinder,and whats on it now...

It's pretty amazing seeing just how many variations GM had on them,yet many will interchange--the main thing is the bolt pattern and push rod seat depth--as long as those match up it'll work,though maybe not "perfect"--thats the beauty of a chevy--most everything fits..

Evolve,you dont have to ditch the hydroboost--lots of GM 1 tons used 292 sixes and V8 gas engines and they all have hydroboost brakes..the only glitch might be finding the right P/S pump bracket for your "new" engine"..many hot rodders are using hydroboost too--my brother rigged a GM one up to his '57 Studebaker pickup ,that has a nova subrame clip and a trans-am disc brake rear end...fits easier than a vacuum booster,works great,and common parts ..
...after seeing what some guys cob together for rat rods with mis-matched axles and brake parts,I guess I'll be OK..

I'm wishing I spent the 6 bucks I got back on a bottle of Jack--for my jacked up back...only took 1-1/2 hours to put the master cyl. on,but it'll take a week for my back to stop throbbing..cant drink though,my stomach cant take much abuse any more either...time for some pain meds and a hot shower...be better if it wasn't 80 degrees with 70% humidity too...it's like a steam bath tonight here...

 

[evolve991]

I know that now Didn't then