What's this under my 78 40 Master Cylinder?

[SliderJack]

Working on my brake lines and noticed this union of some kind under and at the back of the brake master cylinder in my 78 40. What is its function? Can I replace it with a proportioning valve? I have disc brakes stock in the front and replaced the drums in the rear with discs.

Thanks in advance.

 

[brian]

proportioning valve

 

[SliderJack]

Thanks. Looks like there might be a cap on the bottom. Do you know if it's adjustable or am I better off installing an adjustable proportioning valve since I changed the rear brakes to disc?

 

[brian]

not sure how it functions, since it's not a load sensing valve, it just meters out front to back bias at a fixed rate.

 

[antFJ]

I kept mine and added an adjustable prop valve when i added rear disc

 

[SliderJack]

I kept mine and added an adjustable prop valve when i added rear disc

Good to know. Thanks!

I'm replacing all my lines, so if I can do without it, then I'll remove it in favor of the adjustable vale I bought.

 

[sggoat]

just curious, but why replace the valve if it works? Is there something abnormal when you convert to discs in the rear?(seems like the brake system wouldn't care what was on the end of the pipe-discs or cylinders-very simple hydraulics) I'm thinking maybe to convert my 77 to rear discs, so any help here would be beneficial).

 

[SliderJack]

just curious, but why replace the valve if it works? Is there something abnormal when you convert to discs in the rear?(seems like the brake system wouldn't care what was on the end of the pipe-discs or cylinders-very simple hydraulics) I'm thinking maybe to convert my 77 to rear discs, so any help here would be beneficial).

Keep in mind I know just enough to be dangerous, but I think the drum brakes take X amount of pressure to work correctly. This proportioning valve is likely in the system stock so that the rear drums get the right range of pressure. Replacing the drum brakes with discs, changes the system. So if the rear brakes don't operate well in the same range of pressure as the stock system, the rear brakes might lock too easily or not work enough. So, in some cases people add an adjustable proportioning valve so that they can dial in the pressure sent to the rear brakes.

Now I'd love someone to say, in my case of a 78 the stock proportioning valve is all I need, but probably not the case. So I'll have to cut my brand new lines and I think I'll have to get SAE threaded ends and re-flare for the proportioning valve I bought.

 

[Racer65]

just curious, but why replace the valve if it works? Is there something abnormal when you convert to discs in the rear?(seems like the brake system wouldn't care what was on the end of the pipe-discs or cylinders-very simple hydraulics) I'm thinking maybe to convert my 77 to rear discs, so any help here would be beneficial).

When you convert to rear disc, the braking power in the rear has increased, therefore your front-to-rear brake bias is thrown off as compared to spec. The purpose of a proportioning valve is to decrease the pressure in the rear, so that your rear brakes wouldn't lock up during a hard stop situation. The stock valve is designed to provide a specific bias for the stock set up. Once the set up is altered, the stock prop valve won't provide the correct bias. Therefore, an adjustable valve is needed so that you can tune it back to spec.

A secondary reason is that more than likely your stock valve is rusted inside, and not doing a sufficient job even currently.

I offer a solution that has several benefits:
1. It's adjustable
2. It let's you directly plug in your existing brake lines
3. It let's you install it in the stock location
4. Finally...it let's you replace the rusted stock valve that's no longer available!

 

[sggoat]

Thank you!!