The Stovebolt Site Forums The Parking Lot Big Bolts Master Cylinder Frustration

My main frustrations with working on an older truck have come from finding parts, and there are some things that I don't want to get used. Master cylinders, and brake parts in general, fall into this category. In my view, running is good, but stopping is essential. My truck is a 1966 GMC 5500 (HM according to the VIN). In the bigger picture, this is not really all that old (I could have driven it new), but parts for this particular year and model seem hard to find. With that off my chest, here is the situation.

The truck has a dump bed that came with tall racks (mostly bolted together but also welded in a few spots) that limit its use. After removing the rotted plywood, I wanted to move the truck under a tree branch to lift off the heavy steel frame parts. This should have been easy, since the truck was running when I parked it a couple months ago. Starting was not a problem, although the idle problem hadn't fixed itself yet, but then the brake and clutch pedals went to the floor with no resistance - not a good sign. A quick check of the master cylinder found it empty of fluid - also an unpleasant surprise. And there was no sign on the ground where the brake fluid might have gone (hopefully not into the hydrovac or past the wheel cylinders with no pressure on the brakes).

After adding fluid, the brakes pumped up without a problem, but the clutch refused to be primed by pumping the pedal. OK, I already knew the clutch needed adjustment because it didn't disengage until just before the pedal reached the floor. This makes replacing the slave cylinder seem like a good place to start. I was able to find this part on-line. In fact, I found more than one, and they look different. So now it is time to take the old one off in order to know which new one to get.

And since removing the old slave cylinder will probably drain the old master cylinder (again), I might as well replace it at the same time. But this is easier said than done. After checking both retailer and manufacturer websites, no one seems to be carrying, making, or providing rebuilding parts for my 1966 medium duty truck master cylinder, and none of the pictures of dual brake/clutch master cylinders match what I see in the truck.

So where does one go to find a new or rebuilt master cylinder (or rebuilding parts) for a medium duty truck? I am getting used to having to search for parts, but this seems so unlikely that it must be from some shortcoming on my part. The only next step I can think of is another trip to the local Napa dealer and trying to explain what I need to the nice, and smart, young man at the counter who isn't old enough to know from experience what I am looking for.

I am old enough to be cantankerous about having a hard time with things I think should be easy. And this more than qualifies. Any help would be greatly appreciated!

JRMunn

It really depends on how you want to proceed. You can find a Chevvy with a similar weight/brakes and take that master cylinder and use it, or use its rebuild kit. There really isn't much difference in brakes as long as you stay with the same weight class and brake type (vacuum booter, hydrovac, air over hydraulic).

My CCKW has has no less than 4 different hydrovacs over its lifetime.... the latest one (Bendix Series 4) is from the 50's and bolts right up. And this is for a 12,000lb truck. My Chevvy (a 43) is using a late 50's hydrovac also. None of the internal parts are the same, but it puts out the same PSI to the lines.

Have you tried this site:
http://6066gmcclub.com/gmcguy/index.php

I will share with you some valuable info: Midwest Power Products in Morris Illinois. Sole owner is 'Rich' (super nice guy) If he cannot help you, forget it! Reasonable also. He even had a brand new Hydrovac for my 57 2 1/2 ton GMC. Yes, NEW not rebuilt. Phone: 815 942 0550

Pfarber,

Thanks for taking time to reply to my too-long rant. Master cylinders for similar Chevrolet trucks seem similarly hard to find. A problem with mixing and matching is that the dual master cylinder also controls the clutch, so bore and stroke on the clutch side need to be matched to the slave cylinder to provide appropriate travel distance. Strangely, the hydrovac unit for my truck is easier to find than the elusive master cylinder. I am familiar with the GMC 6066 Forum where I have learned much to help with my 1964 GMC pickup. It looks like I will be going to the local Napa dealer and hope for the best.

JRMunn

Old GMC Gold,

Thank you for identifying a potential parts source. I will be calling Midwest Power Products first thing on Monday morning. Maybe they can help me get the correct (or at least matching) slave cylinder as well.

JRMunn

Hy JRMunn, I see what you mean about availability, I did a bit of searching. The O.E.M. p/n 5467823 that equated to Wagner FE56462 which appears to be no longer available, but it did tell me that the brake side cylinder bore was 1 1/4" and the clutch cylinder bore was 1 1/8". The Raybestos catalog had no listing for your application, however some detective work led me to their p/n MC36347 which is listed as the master/clutch cylinder for a 1968 T50 and it is the same O.E.M. p/n (5467823) as the master/clutch cylinder for your truck. Napa crosses the Raybestos p/n to their p/n NMC P55385 $55.99 plus a $33.00 core, hope that helps and Merry Christmas.

3B,

Thank you! That is great information. I need to learn how you do it. I am going to look up the Raybestos master cylinder to see if there is a picture.

JRMunn

I got my 43's 'vac from Midwest. Great guy to talk to. Prices are also very good. He even offered to look at my Bendix Series 1 and see about rebuilding it.

I have an old Napa catalog but it doesn't show anything for 66 except air brakes. There is nothing wrong with putting kits in as long as the wear area of the cyl. is not pitted. The clutch part is usually OK as it takes a full stroke each time.
Also if you have the time you can send the cyl. out & have it sleeved in stainless steel or brass.

I talked to Rich at Midwest Power Supply by phone this morning. This was much simpler than a trip to the Napa Auto store. I agree that he is good to talk to and is very knowledgeable. Better yet, he has the now out of production master cylinder used on my truck in stock (with correct brake and clutch bore sizes). Price was high compared to other units with different bore sizes that would fit the bolt pattern, but I decided to get the "stock" unit to avoid unknown (to me) effects on the clutch and hydrovac performance.

Since these parts seem to be going out of production, and I plan to keep the truck, I will look into rebuilding and keeping the cylinders I take off so that I, or a future owner/user, does not have to go through this or worse in the future.

Thank you to everyone for your answers to my parts problem. I could not have found this master cylinder without your help.

JRMunn

Yeah, I forgot to mention that Midwest can resleeve/rebore your old wheel cyls. and master cyls.
I was very pleased with the parts from him, including the Hydrovac. It's unreal how my truck stops now, just like modern power brakes. Of course it only cost me $1000 to replace everything including lines, but worth it.

I want to finish off this thread with what finally worked, so that others might not have to repeat my experience. I eventually found a tandem (brake & clutch) master cylinder that would work. It is a Raybestos MC36347. This is not listed for my 1966 GMC HM5500 truck, but does fit and has the needed 1 1/2" brake bore and 1 1/8" clutch bore. The clutch slave I got is a Centric 138.80003. There are Bendix products that also appear to fit.

Unfortunately, it seems that both the Raybestos and Bendix master cylinders are now out of production. I got the Raybestos part from old stock being sold for a great price ($100 with free shipping) on Ebay. So don't delay if you think one of these will be needed in the future. I found the Centric clutch slave at an on-line Porche parts dealer. It seems to still be in production.

Then I found out that the master cylinder does not come with new dust boots, and the ones on the truck were completely rotted away. These fit against and through the firewall, rather than on the master cylinder, and were impossible for me to find. Eventually, I came across some workable replacements in the cardboard "boot box" at a nearby power brake and hydrovac rebuilding business.

After getting new steel and hose lines put together for the rotted out clutch lines (including some cutting and flaring of my own to re-use the custom bent line going into the clutch slave), I was able to put it all back together. Then I found out that air would not leave the clutch line with the usual push and hold the pedal and open the bleeder valve (many times) procedure. So I broke down and bought a Mityvac bleeder, which eventually worked after I put Teflon tape around the bleeder valve threads so there would be enough vacuum to pull fluid through the lines.

Both the brakes and clutch are finally working again, and I was able to drive (meaning move) the truck last week. In the process, I also found and repaired loose and leaking vacuum lines to the hydrovac and to the two-speed rear end switch, and there was a major vacuum leak where a hole had been burned through the manifold vacuum supply hose (at the back of the engine) following a previous owner's repair that pushed it against an exhaust manifold (so the truck also idles better now). I was doing this in a field during the California drought. Now it is raining (thankfully), and I will have to wait until the ground gets firm to drive the truck under a tree to remove the steel rack frames, which is finally back to what I wanted to do a month ago.

I hope this helps, or at least forewarns, a future shade tree mechanic's clutch and brake repairs on a mid-60's GMC medium duty truck.