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Braking issue

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shades_1970

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May 28, 2009
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Location
Larchwood, IA
'76 K5 with '76 3/4T running gear(D44/10.5)

I recently got my rear brakes working(13"x2.5"), then the master cyl blew out. Having most an '87 C10 sitting around rusting away, I pulled the booster, master cyl, prop valve, and lines between the two and installed them in my K5.

I have SOOOO much rear brakes now that it is scary in some situations...:eek1:

FINALLY, the question...Is the plumbing after the proportioning valve 'reversed' after '80 so the '84 master cyl putting the massive front brake pressure to the rears?
 
I have similar issues with my '82 K250 now that I used a rebuilt master cylinder on it I had from one of my previous trucks...my original master cylinder had a square casting with 2 fluid chambers ,one large one up front,and a smaller one to the rear --the rebuilt cylinder I used had an oval shaped casting and had two equal sized chambers for the fluid...

The steel lines on my original master had to be swapped from front to rear to fit the "new" master cylinder,as the thread sizes were diferent...the push rod seat was the same depth and it bolted up OK (my truck is diesel with hydroboost brakes,the master I used was off a vacuum booster gas powered truck)....so in other words I had to put the steel line that was "up front" in the rear chamber's outlet and vise versa..

Now ,after bleeding the brakes ,they work fine--TOO good in fact,just like you stated--the rear wheels,especially the left one,will skid way too easily,its dam near impossible to make a quick stop without that wheel locking up now....but I also have brake shoes that had been soaked with brake fluid and gear oil and all I did was wash them off good with gas and brake cleaner..should be replaced but I lacked the cash,thats also why I used that other master cylinder I had lying around..

I'm unsure if buying the adapter fittings to let me put the steel lines back to their original locations will help any--many GM trucks I have owned had real "grabby" rear brakes even with 100% correct factory parts,especially the ones without that load compensation device on the rear axle that limits rear brake line pressure (my 82 dont have that)....I've also noted the rear chamber on this master is slowly losing fluid just like the original one did now--despite the fact it was stored in a dry shed with both chambers full of brake fluid...it might have torn a piston seal while I was bleeding the brakes?--I see no evidence of leaks from any of the brake lines or wheel cylinders (so far,haven't given it a real good look thanks to it being cold and snowy this winter)...so I may end up having to replace the master cylinder again anyway...

But if you find the problem with your brakes ,I'd be interested to know if it was due to the lines on the master being hooked to the "wrong" ports....
 
There are a lot of different things going on here.
If the chambers are different sizes, then odds are its for a front disk setup.
Disks need more volume than drums.

But, disk/drum masters are different from drum/drum masters in other ways too.
First you have residual pressure issues.

Some setups leave a small amount of pressure on the lines when you release the pedal to keep the shoes close to working so you get a shorter pedal.

That amount of pressure is different for disk vs drum.

Also, drum brakes are "self actuating", whereas disk brakes are not.

A drum brake uses the wheel cylinder to engage the shoes to the drum. Then the spinning drum rotates the shoes so they wedge tighter. Thus the drum does most of the work making it easier to stop.

Due to the smaller size of the wheel cylinder, drums take less volume than disk.
Disk brakes take volume as well as pressure. So, if you have a disk master driving a drum, then you will move many times the volume per distance of pedal movement than it needs to work.

Which is going to make them very sensitive.

Whereas with the right master, you might move the drum wheel cylinder 1/4 inch per inch of pedal movement. ( Just pulling numbers out of the air, no idea of the actual ones.)
With a disk master, you might move the cylinder 1.5 inches per inch of pedal.

If it only takes 1/2 inch to lock the wheels, you can see how sensitive it would make it.
 
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