On Going Brake Issues

[KyleSeal]

Hello all.

Im working on a 78 GMC C25 (2WD) front brakes.

The issue started when the customer described to me that the front left felt 'grabby' at higher speeds. The red brake light on dash was on.

By grabby, what we are saying is that it felt like an ABS issue on a modern vehicle, and like the brakes were off-on-off randomly, causing a front left nose dive. I suggested doing pads, rotors, calipers on the front as the old stuff was crusty, and rotors had heavy grooves. Taking apart, found that the front left caliper was leaking from the piston at some point, possibly active. My guess was that this was causing the red brake light, as in the prop valve had been triggered (possibly causing this 'grabby' issue?).

So I proceeded, checked hose flow - good. Replaced pads & rotors + calipers with some off the shelf units. Reset prop valve with the bleeder tool I had, red brake light on dash is now off BUT the brakes are non-existent.

When I bleed the brakes, I am getting a ton of air, and I mean a ton. I feel like Ive run two quarts through the front brake lines and still, air, and lots of it. I even replaced the bleeders with Speed-Bleeders so I wouldn
t have to bother anyone to bleed with me. I did notice the bleeder on the passenger side was loose at about 1/8th turn, and putting fluid out, possibly pulling air in between pedal strokes?

I have checked, no leaks, lines are in good shape, all connections are dry, prop valve tool is in when bleeding, calipers are bleeder @ top, and I have no idea where to proceed from here.

What gets me the worst here, is that before this I felt like the pedal was reasonably good, small bit of dead-travel, but honestly better than most I've driven. But now the pedal is just zero resistance.

Is it possible whoever drove this before I got to working on it ran the master dry and killed it? Im at a bit of a dead end of my expertise with this issue.

Thank you for any help.

[Bandit130]

My suggestion is to replace both front flex hoses and master cylinder if you don't know when they were replaced previously. The hoses can expand internally causing less flow on one side or the other. If you have a left nose dive, the right flex hose is bad. Right nose dive, left flex hose bad. In a lot of cases, the left can go bad before the right. I had a slight left pull on mine, the left hose was previously replaced but not the right. My master cylinder was contaminated causing brake light to be on. I replaced master cylinder, flushed lines, replaced both rear brake cylinders and all was great. BTW, gravity bleed worked well for me.

[406flyfish]

Pull the master and bench bleed it. Then You’ll know if it’s bad or not. I had terrible issues getting mine to bleed out. (78 k10) it had very little pedal and I got good fluid but couldn’t get the air to clear. I ended up pressure bleeding it and finally got all the air out.

[Corts60]

I agree with above. Replace flex hoses and master cylinder. Then I would also suggest using a vacuum bleeder if you are doing this by yourself.

[Dead Parrot]

2nd the replace the master cylinder thought. If the bore had any rust from the piston never getting that far during normal operation, your going full stroke during the bleeding could have damaged the seals and resulted in your lack of pressure.

Check along all of the metal lines. Salt can corrode the metal lines to the point fluid gets out and air gets in.

If the fronts were that bad, I would at least check the rears. You don't want to send your customer out with bad rears after you get the fronts fixed.

[KyleSeal]

Well, a bit of an update here. I did a very methodical bleeding of the front of the system to just pin down where my issue is. Bled at master cylinder, once I had a steady amount of fluid, moved to the calipers and got those bled again. I was able to get a strong pedal.

Heres the kicker.... noticed brake fluid at the bottom of the booster. I believe the master is leaking in the rear, as some have mentioned as a possibility. Im hopeful it's just the cap leaking, but Im not that hopeful. Im going to pull the master on lunch today and see what I can find out. Hoping it hasnt filled the booster with fluid.

[KyleSeal]

another quick update - drove the truck, brakes feel pretty good. I will probably still elect to replace the master based on the condition on the inside of the reservoirs.