Brake help! Exhausted ideas...
 

Hello everyone and thank you for taking the time to read this.

Brake issues on my '70 GMC. Manual 4 wheel drums, dual-chamber m/c.

Have had multiple issues with the brakes, have replaced everything that came up, and I'm still stymied.

Hard pedal, almost zero front brake action, when it does grab a little the front right grabs more. Cannot get a good bleed from the front brakes.

SECOND New mastery cylinder, properly bench bled (first one I did I might have messed up during wheel bleed by bottoming it out, but I never got the front brakes to really work with it, either). New wheel cylinders all around. New shoes all around. Front brake soft line sections replaced.

My understanding is the "combination valve" below the m/c, even if failed, will not impede the flow of fluid. But that is the last component (other than the hard lines) that I haven't replaced, along with the soft line that runs over the differential.

Do I need to consider a possible clog in the hard lines to the front? What else?

Re: Brake help! Exhausted ideas...
 

Does your new master have residual check valves in the outlet ports?
Combination valve is just a distribution block so shouldn’t be a problem unless it’s tripped.
Did you centre the valve while bleeding?

Re: Brake help! Exhausted ideas...
 

I will go see if it has residual check valve ports.
What if the distribution block has tripped? will that actually prevent fluid from flowing through the "failed" side?
Did I center which valve?

Re: Brake help! Exhausted ideas...
 

Afraid I'm not a lot of help but I'll toss a thing or two in just in case.

With all drums, you do not have a combination valve (aka proportioning valve). Those are used to vary line pressure between front & rear when you have front discs. What you have is a distribution block.

Is there a good flow of fluid when bleeding the fronts? If so, the lines are probably not blocked. If not, try disconnecting the front brake lines a section at a time, maybe starting at the MC itself. Check for fluid flow at each place you open the line. Something has to be blocking the flow and this should help find the blockage.

Good luck, hopefully you'll get more suggestions, and keep us posted.

Re: Brake help! Exhausted ideas...
 

No it doesn't have a residual check valve.

Yes, I understand it does have a distribution block and not a proportioning/combination valve. I just keep mixing the terms up. I've also read it being called a "pressure differential valve" or "brake pressure transducer." I get that the proportioning is done entirely by the M/C in a 4 wheel drum setup. And my impression is that even in a failure state, the distribution block shouldn't be able to impede flow. Though I am not 100% sure of this.

I think your advice makes sense at this point. And no, I'm not getting really any fluid flow from the fronts.

Re: Brake help! Exhausted ideas...
 

What do you mean by "Cannot get a good bleed from the front brakes?" What is happening? Are you doing the bleed with a helper or using a one-man bleed kit, or vacuum bleeder, or what?

Re: Brake help! Exhausted ideas...
 

There's almost no movement of fluid. Sorry to not be specific. With a helper.

Re: Brake help! Exhausted ideas...
 

OK, that gets back to opening the front brake line at various points to determine where the flow is good and where it is not. Start at the MC, open it there, see if you have good flow leaving the MC.

Re: Brake help! Exhausted ideas...
 

The distribution block can trip either way if it’s not held in the center position while bleeding. Result would be little fluid flow to whichever side is blocked.
You can’t check for residual check valves unless you pull the lines off the master. They’re inside the ports.

Does your distribution block have a reset button on the front?
Some do.
A pic would help.

Re: Brake help! Exhausted ideas...
 

I see. But the residual check valve just maintains a little pressure in the line to help prevent air seepage, no? It's not related to a failure like this, is it?

How would one reset or center the distribution block's piston if it were tripped? Is there any way to verify that it was? It's a part that I can't even find replacements for anywhere--even LMC doesn't identify it. the Haynes manual makes no mention of its existence or needing to keep it centered during a bleed, either.

Re: Brake help! Exhausted ideas...
 

Attachment 1925216
Does not appear to have a reset button.

Re: Brake help! Exhausted ideas...
 

Correct on the residuals. I mentioned it because without them you’ll have poor braking.
Some dist blocks have a reset. Some you remove the switch and centre the valve.
Like I said, a pic would help.

Re: Brake help! Exhausted ideas...
 

I just posted a pic above. Are you able to see it?

Re: Brake help! Exhausted ideas...
 

One trick people say works is slamming the pedal down hard to centre it.
You can remove the switch and centre the valve using a tiny screwdriver. It shouldn’t leak if the seals are good.

Re: Brake help! Exhausted ideas...
 

Yup!

Re: Brake help! Exhausted ideas...
 

Worth a try. Are you absolutely sure that a failure state in this thing will actually impede fluid flow? I've read that it won't, I've read that it will, but it's all just people talking on various old threads and nothing concrete.

Re: Brake help! Exhausted ideas...
 

I can’t say for absolute how much it stops.
It’s all voodoo!
JK!!

Re: Brake help! Exhausted ideas...
 

Worth a try for sure. Press on the brake pedal very very hard. The pin just might center itself.

I just learned something too, I knew an off-center pin would light the brake warning light on the dash but I didn't know it would restrict fluid flow.

Re: Brake help! Exhausted ideas...
 

I suppose the best way would be to trip a valve and pull off the outlet line on that side and see how much flow you get.

Re: Brake help! Exhausted ideas...
 

I just tried putting pressure on the valve's piston with a screwdriver after removing the switch. There was a little initial seepage, didn't try stepping on the brakes with the switch removed. Didn't feel like I was able to move the piston. I tried stomping on the pedal pretty hard, but I guess I could go harder.

Rears still locking up almost immediately, fronts doing jack.

Re: Brake help! Exhausted ideas...
 

Piston needs to slide sideways.
You might need to loosen a line on that side in case it’s pressured up between the master and dist block.
Or remove the outlet line from the dist block to the front brakes and see if the piston can be moved that way.

Re: Brake help! Exhausted ideas...
 

If you get good flow out of both sides of the master cylinder, I would bet there are tons of those distribution blocks people have laying around after their disc brake conversions and you could ask for somebody to send you one, and that may solve it. If you become a premium member you can just post a want to buy ad. That's what I do when I need some obscure part. The membership is a good idea.

Re: Brake help! Exhausted ideas...
 

They make this little plastic plug that screws into place of the switch.
Used to keep the valve centered while bleeding.
Parts store might have one.

https://www.summitracing.com/int/par...tool/overview/

Re: Brake help! Exhausted ideas...
 

So, I have good flow from the distribution block's front outlet port. I also just rechecked the front right bleed at the wheel cylinder bleed screw, and I have good flow there too. So now I'm stumped. The pedal feels good, I have flow in the front of the system. I couldn't check the front left wheel at the cylinder; I couldn't get an open or socket wrench on, there's literally no space unless it had some kind of weird offset.. I could scarcely get a wrench on the right one--I forked the bleed screw with an open 3/8" and used vice grips on the end of the wrench to rotate it.

But if I'm getting fluid pressure sufficiently to the front, only thing I can envision is bleeding the whole thing again.

Re: Brake help! Exhausted ideas...
 

Like these... https://www.toolsid.com/vim-tools/br...yABEgK1H_D_BwE