Hard brake line question

[Yukon Jack]

I am swapping a power brake master cylinder from a 78 3/4 ton 4wd to my 70 K5. The 78 proportioning valve uses smaller diameter hard lines for the front brakes as compared to the 70's front hard brake lines from the proportioning valve to the brakes.

Can I tie into the proprotioning valve using the larger existing front lines or do I need to replace them with the smaller diameter hard lines?

[cableguy0]

i would use the smaller lines the bigger lines will most likely cause a pressure drop and your brakes wont work properly disk brakes use a lot more line pressure to operate than drums do

[Tx Firefighter]

Use adapters. That's what everyone else does.

Pressure will not drop. The pressure in a hydraulic system has nothing to do with the size of the line. The pressure is influenced by the size of the master cylinder bore. Those lines could be 6 inches in diamter and the pressure would be the same.

[poff]

I believe disc brakes use more volume of fluid not more pressure than drums. Why not use a '71 or '72 booster? Should have the same master and the booster bolts up. I believe you can use the late model prop valve and plug one hole. When I converted my '70 I got booster brackets, a stock prop valve and hard lines for a '71. Everything bolted up. The parts were not too expensive. We did the same thing to put 3/4 ton brakes on my buddies '69 K-5. If you call Inline Tube and tell them you need brake lines for a K-5 with 3/4 ton brakes they can mix and match the lines to fit you truck. FYI: On a 1/2 ton the front reservoir goes to the front brakes and on a 3/4 ton the front reservoir goes to the back brakes.

[Fred T]

Tx Firefighter is right. The only thing a larger line does is allow more fluid to move. No change in pressure. Go to the parts store and get some adapters.

[Tx Firefighter]

Here's a picture of my truck when I first converted it. I used adapters as you can see if you look close at the 2 lines coming out of the prop valve.

I just got a 72 master cylinder at Vato Zone and a used prop valve off the parts board. I used the 2 adapters and bent the lines to where they would screw in.

It worked very well, but I wasn't satisfied with the looks, so I later built all new lines fron scratch so they were bent just so and had no adapters. The only reason I did that was because I'm vain and it looked ugly to me.

[neonlarry]

I think the reason GM used the smaller lines was cost. Unlike 72 and earlier there are two line for the front brakes right out of the P valve, 72 and earlier used one line to a T on the frame. Are you putting discs on the 70? If not I would not use the 78 P valve, it is set up for disc/drum, unless you put a 10lb residual pressure valve inline with the front brakes. Other than that use adaptors like TX said.

[Yukon Jack]

I bought a 78 3/4 ton parts truck for the axles and scavenged the master brake cylinder and have it on the 70 K5 now. Thanks for the above info, that really helps.

[Yukon Jack]

As stated, the 78's proportioning valve I am using has an out for the front left and front right brake lines where as the 70's brake lines go with one line down to the frame and then it splits into left and right.

Can I plug one of the front outputs on the 78's proportioning valve and just use the existing single front line and allow the split to left and right be down on the frame like it is already? That will save me having to run at least one new front line.

[Palf70Step]

I'm late, par usual...I guess that';s from being an inliner.

Yukon, if your still using the orginal brakes, why not use the new master cylinder, but keep the old lines & proportioning block. You shouldn't need the new one if your staying with the original brakes. No real reason to swap it out unless your changing it over to discs.

[Yukon Jack]

The 78 axles does give me front disks and the 14 bolt has those big lovely drums. So I need to stay with it's proportioning valve.

[Palf70Step]

OKJ...Just put my old self out to pasture.

Brain is slow these days.

I am pretty sure the newer proprtioning valve needs that split at the valve to permit it to work correctly. One of the ones on my dauughter's 82 got clog and the pressure imbalance cause the valve to go out, so I think you plugging one would have the same effect.