I have a need to get an old 1951 GMC 2.5 ton side-dump farm truck
running for a few errands. This truck has been parked out behind the
barn for about two years now.

My question is about a part in the brake hydraulics system. When I
started the project, there was no brake pressure. A several minute
search brought me to the master cylinder that is located under the
driver's position floorboard. Pulled a plug from the floor and found
where the reservior is (it is actually the body of the master cylinder
itself).

Following the hydraulic line from the master cylinder brings me to the
part in question. This is a large cylindrical device about six inches in
diameter with a narrower neck coming off the front of it. The neck feeds
into a rubber tube that ends at a breather of some sort mounted on the
firewall. There is a bleeder for the brake system in the narrow neck as
well, and the brake lines come off of there too.

There is also a feed from this device to the hydraulic pump for the dump
bed on this truck, but I don't believe that the system shares the fluid
between the brakes and the dump hydraulics.

I got the brakes working well, but I'm still baffled by the purpose of
this rather large device in the middle of the system.

Any ideas?