Fitting size to hook up rear brakes

[Pender1]

I've managed to FUBAR the fitting that connects the hard line on the frame to the flexible line that runs to the rear axle. I'm hoping I can just cut off the old one and flair the pipe with a new one on it, but can anyone tell me what size fitting I need?

Right now this is the only thing stopping me from getting my truck back on the road and it's driving me crazy.

 

[chengny]

Brake lines are either 3/16" or 1/4". The difference is usually obvious - unless they are wicked rusty. All the brake lines on your truck are 3/16" OD - except the one you are working on.

That common line for the rear brakes that runs from the proportioning valve, along the inside of the rail and then is coupled to the axle flexhose - that is the only section of 1/4" tubing in the system. There is actually another one - it the runs from the master cylinder to the proportioning valve. The other line that runs parallel with it is 3/16" - you could use those as comparison:

You must be registered for see images attach

Yours might be reversed.

Another easy way to gauge the OD of a brake line is with a 1/4" open end wrench. It will fit easily onto 1/4" line and will bang back and forth on 3/16" line. Like this:

You must be registered for see images attach

If you are ever unsure of what size tubing or fitting you need, a good way to make certain that you get the right one (without making several trips to the parts store):

Just cut the fitting off (or a small bit of the tubing) and bring it with you to the parts store. I do that all the time when working on European cars.

 

[89Suburban]

Great post

 

[Pender1]

Brake lines are either 3/16" or 1/4". The difference is usually obvious - unless they are wicked rusty. All the brake lines on your truck are 3/16" OD - except the one you are working on.

Are the fittings a standard thread based on line size, or are there different threads for a certain sized line?

 

[chengny]

For line fittings, there are two standard thread/diameters. One for 3/16" line and one for 1/4" line. But do be aware; there is a long style and a short style of each one.

1. A1-3 is for the 3/16" tubing. It measures 3/8" by 24 unf.

2. A2-4 is for 1/4" tubing. It measures 7/16" by 24 tpi (threads per inch).



There is however, another series of tubing nuts/fittings that use larger OD's. These are called "master cylinder" nuts. As the name implies, they were originally used to join standard 3/16" and 1/4" tubing to the master cylinder.

There are three m/c nut sizes available for 3/16" tubing:

1. 7/16" x 24 tpi

2. 1/2" x 20 unf

3. 9/16" x 18 unf.

For 1/4" line there are two sizes of m/c nuts:

1. 1/2" x 20 unf

2. 9/16" x 18 unf.

Over time these nuts began to be used elsewhere in the brake piping system. Now you never know where they will pop up. On your truck for example, in addition to the MC, you will find them used to connect the supply and outlet lines to the proportioning valve:

You must be registered for see images attach

They might also be used on the load compensating valve or elsewhere in the rear hydraulic circuit. I don't think they are and you shouldn't have to be concerned about them - but since I am not really sure what your exact repair intention is, I just thought I would mention their existence.

Trust me on this, the best way to be sure you get the correct fittings you need (with just one trip to the parts store) is to bring the old fitting with you.

 

[Pender1]

Thank you very much. I'm going to run by the parts place tomorrow and see what I can do. One step closer to a running truck.