47-55.1 Reversed Brake lines!!

[vintovka]

Been having an ongoing issue with the brakes on mine. Its on an 84 k10 chassis with all stock parts. Brakes work but cannot lock them up if needed. Work was done by a paid licensed "professional restorer".

Just added all new brake parts , hoses, proportioning valve etc with minimal improvement.

Today a local mechanic reported that the brake lines were reversed at the master cylinder. Awaiting his return with a double flaring tool to change fittings to "correct" the situation. Pic shows lines the way they were installed

Might have a word with the "professional installer' What do you think?

[jweb]

I think most master cylinders the front line goes to the front brakes. You may want to research yours before moving any lines, I think there are some that work the opposite.

If you still have issues, I would recommend adding a proportioning valve


You could also use an adjustable valve like this:
https://www.speedwaymotors.com/Adjus...Query=91031367

[vintovka]

Currently front line goes to rear and vice versa. this was discovered when a new proportioning valve was being installed. Current mechanic was floored when he saw was wrong. Had there been an accident someone would be in deep.

[joedoh]

Originally Posted by vintovka

Currently front line goes to rear and vice versa

this is how modern chevy trucks are plumbed, at least every s10 and c10 1988-2004 i have worked on

[dsraven]

usually that would be a cast iron master cylinder/reservoir and it would be relatively easy to see which is front and which is rear by the size of the reservoir attached to each circuit. the front brakes being discs would use the larger reservoir. I think I have seen the masters plumbed both ways before, with the stock cast iron master. just ensure the prop valve is plumbed correctly as well.
do you have a stock combo valve in the system? if so, it may need to be reset after bleeding the brakes because the piston in the cylinder of the valve shuttles to one end and closes off the output for the other end. maybe this would help?
https://www.chevytalk.org/fusionbb/a...cf4f54b6ed9df/

http://www.classicperform.com/TechBo...roubleshoot.ht

[MiraclePieCo]

I reversed my combo valve so the front line goes to the front brakes and vice versa. No sense in having the lines criss-cross one another.

[Rickysnickers]

Originally Posted by MiraclePieCo

I reversed my combo valve so the front line goes to the front brakes and vice versa. No sense in having the lines criss-cross one another.

That's nice and clean looking. W/o killing this thread, can you PM what you did for that oblong steering wheel hole, please?

[1project2many]

Hold on.
On this master cylinder:


The rear port is designed to be connected to the front brakes. This is a "quick takeup" master cylinder. It was used on many, many, many GM vehicles. It's easy to tell it's a "quick takeup" disc/drum system by looking at the bottom of the m/cyl. The back will be larger than the front. This system works with specially designed calipers which have seals that retract the caliper pistons a little more to reduce drag on the rotors. The "quick takeup" portion is designed to pump a large amount of fluid into the calipers at low pressure when the brakes are first applied, then the master cylinder automatically switches to using a smaller bore piston to apply high pressure for stopping.

Here are some links about quick takeup systems:
https://www.freeasestudyguides.com/b...-cylinder.html
https://slideplayer.com/slide/4616100/
(quick takeup begins at 17:00)

When I first learned about these systems I was excited. They seemed like a great idea. But in practice they absolutely suck! All of the fluid from the low pressure "quick takeup" portion of the m/cyl flows through a small area to get to the front wheels. The system doesn't apply significant pressure to the rear brakes until there is pressure in the front brakes. So if you hit the brakes fast you're met with a slow and sluggish brake pedal and a vehicle that just doesn't want to lock the brakes quickly. What a great response in traffic.

I've tried replacing a vacuum booster with HB and it helped some, but the damn quick takeup m/cyl was still there and that sluggish pedal was still present. With my latest DD I just switched to a standard bore m/cyl and tossed out the quick takeup m/cyl all together. What a difference!!! The pedal apply is as fast as you want. I can lock the brakes so fast the engine damn near leaps out of the front of the truck. And the dire warnings about not using a standard m/cyl may be overblown hype. In my case the pedal is a little low, but it's no worse than a stock pedal if the rear shoes need adjustment. So I call it a win.

[vintovka]

What you described is EXACTLY what is going on with mine. I have standard OEM front calipers and drum rears. Is there a part number for a regular master cylinder that you describe?

[1project2many]

Sort of. I had changed the front brakes to a larger system from a later truck. I picked the master cylinder based on the caliper and master cylinder bore sizes. The application was a 1980 Camaro Z28.

Do you know what the original application is for the parts you have now? We could probably come up with one. I'd consider a 1" or 1-1/8" bore to start.

[vintovka]

Everything was stock for a 1984 GMC K1500 (chev k10) with 350.

[1project2many]

Hmm... You know, I should have remembered the Y/M/M.

NAPA is showing several different models for an '84. Some are aluminum and some are steel.
The steel type are listed as 30 mm bore which is about 1.8" and there's an aluminum style listed as 1.125". That's a really big difference. Looking at older models that have no chance of the quick-takeup I can see that a 1.125" m/cyl was used with both 1/2 and 3/4 ton trucks. But GM got tricky because on heavier trucks they used a caliper with a 3.157" bore instead of the 2.94" bore part used on half ton trucks. Both calipers share external dimensions.

If I were in your shoes I'd consider going to the 1980 master P2072 with a pair of larger 3-5/32" calipers from a 3/4 ton truck. That should make it stop with mild pedal effort.

What's tough is that as a pro mechanic, much of the diagnosis for brake problems involves putting my foot on the brake pedal. There's a lot of information that an experienced guy will pick up that someone else may not. I don't like recommending spending $$ to replace parts without having confidence they will help. So as far as the sluggish pedal, I can only hope this is the cure.

[57tailgater]

I'll be watching this thread as I thought the port closest to the pushrod was for the front brakes and the other one was for the rear. This way pressure was applied to the front brakes prior to the rear so the rears don't lock up and spin you around. If you pull the master cylinder apart, there's a spring in between the plungers inside that allows this to happen. The block is a metering block to balance the pressures appropriately given the engineered application. If you're not getting your brakes to lock up or stop as quickly as they should, there's probably an issue with master cylinder bore sixe, wheel cylinder bore size, disc vs drum brakes, pedal ratio or other things. I may be all wet on this and if i am, I am open to being corrected by the facts.

[vintovka]

Well will be after Turkey weekend before any updates. I emailed this thread to my guy on this so he may chime in. Been with me thru this whole "post restoration" dilemma ordeal. May eventually look at replacing MC with the larger recommended one and see what happens (funds permitting).