Disc Brake conversion - can’t get pedal pressure

[65_Satellite]

Just throwing out my experience bleeding systems over the years, they suggest bench bleeding for a reason. The brake pedal won't bottom out the piston in the master cylinder like it will when doing it out of the car. If the piston wasn't bottomed out when bench bleeding there still could be air trapped. (You might have done this) Sometimes on rare occurrences air can find it's way out over a couple of days.

The 15/16" bore master cylinder creates more pressure at the caliper and wheel cylinders than the 1" bore. The thing is you'll have a bit more pedal travel with the 15/16" bore. Maybe up to an inch more than the 1". Using a synthetic fluid like Dot 5 will create even more pedal travel. (That's why I asked what you were using).

The way I check for a bad master cylinder (internal leakage/bypass) is to pump up the pedal till it's hard and hold the pedal firmly. As you're doing this, slightly release pressure and feel if the pedal starts creaping down. If you experience this then it could be leaking internally.

Of course peaking under the boot at the rear of the master cylinder and inspecting it for fluid leaking is a sign of a bad one also but in this case the master cylinder could be operating properly.

Another I have to ask...You do have the front brake line hooked to the large reservoir? (Not easy to do with most master cylinders having two different size fittings but you never know).

The brake pedal rod needs a little play just enough that it's not applying pressure on the piston not allowing it to return to it's most rearward position. That's another thing to check.

These are all great points. I didn’t realize that about the brake rod not fully depressing the MC when in the car. I guess I’ll just pull it out and start over. As for the brake rod, I’m using my original from the single reservoir that came with the car simply because it has the protrusion that hits the brake switch. When the pedal is fully released, it is pretty tight and I can’t really move it. Maybe that is causing the master not to fully release? I guess I can try using the adjustable one and if that makes a difference then I’ll install an inline pressure brake switch under the hood

 

[hunt2elk]

And would that be an issue? Here are the two ports
View attachment 1105127 View attachment 1105128

I really don't think it matters. You can pull it out by turning in a sheetrock screw and then pry it out. I went through this several years ago with a mc I bought from Pirate Jack's who sourced them from Wayne the Ram Man. I got to deal with him because PJ had no idea what was going on. Of course Wayne was adamant there was no problem because of how arrogant he is. After a couple days he called me back saying I was right. The whole current batch of mc's he had, it was like 200 units, had bad residual valves. He had to replace them in everything he had as well as get ahold of the people he already sold them to. After that fiasco 7 years ago, I buy all my brake stuff from Cass. I'll pay a bit more for his customer service and technical help.
I have a sneaky suspicion that your issue is inside you master. Either it is leaking internally, or like stated above by KK, I think you didn't get all the air out by bleeding it in the car. It's that last little bit at the very end of the stroke that will get the last of the air out. Don't know if that is always possible when hooked up to your pedal assembly.

 

[khryslerkid]

I agree and I usually do that but for some reason ended up mounting this and after squeezing under dash to tighten the nuts I realized I didn’t but I didn’t want to remove it at that time

This more than likely is your problem. Good luck!

 

[4mulas]

I had same problem with entirety purchased from Dr Diff. 15/16 master, rear brake cylinders etc... Cass said I needed to cut my rear brake line and install, I believe a 2lb, residual valve. This requires a flaring tool to do. I had brake shop do it. That was with a new Dr diff master cylinder and new rear brake cylinders.

@diesel_lv , Did it cure your problem?

 

[4mulas]

I’m aware of the newer wheel cylinders having residual valves but I didn’t see any mention of the OP installing new ones.

Newer Wheel cylinders don’t have residual pressure valves built into them, they are supposed to have cup expanders though (a springs which forces the cup out against the bore of the cylinder) which are supposed to eliminate the need for the residual pressure valve on the rear circuit of a disc/drum set up using an newer style master that has no residual pressure valve built into it.

 

[diesel_lv]

@diesel_lv , Did it cure your problem?

No it did not. I am still battling it. More lazy than anything else in this heat. Thinking it's somehow getting air in the master cylinder. Has brake shop bleed whole system. I still have to pump brakes to get them firm, then all of a sudden I'll go to apply and they are firm at the very top. They'll stay that way awhile and then all of a sudden I have to pump them again. Totally stumped.

 

[4mulas]

No it did not. I am still battling it. More lazy than anything else in this heat. Thinking it's somehow getting air in the master cylinder. Has brake shop bleed whole system. I still have to pump brakes to get them firm, then all of a sudden I'll go to apply and they are firm at the very top. They'll stay that way awhile and then all of a sudden I have to pump them again. Totally stumped.

Hmmm..... did your shop install a 10psi residual or 2 psi residual like you said?

 

[diesel_lv]

Hmmm..... did your shop install a 10psi residual or 2 psi residual like you said?

2 psi which was the recommended one from Cass.

 

[bc3j]

No it did not. I am still battling it. More lazy than anything else in this heat. Thinking it's somehow getting air in the master cylinder. Has brake shop bleed whole system. I still have to pump brakes to get them firm, then all of a sudden I'll go to apply and they are firm at the very top. They'll stay that way awhile and then all of a sudden I have to pump them again. Totally stumped.

FWIW Another possible cause is the master cylinder bore size is too small for system requirements. I ended up going from 15/16” to 1 1/32”.

 

[4mulas]

2 psi which was the recommended one from Cass.

I just spoke with him 2 days ago and he said 2 psi is for front and 10 psi is rear. (The 2 psi valve is mostly for hot rods where the master is below the floor and the inlets of the caliper lines.) You may want to email and ask or phone and ask him again.. see the link for some facts and clarification...

https://www.ecihotrodbrakes.com/brake_facts.html

 

[4mulas]

FWIW Another possible cause is the master cylinder bore size is too small for system requirements. I ended up going from 15/16” to 1 1/32”.

15/16 isn’t too small for the system at all. Yes 15/16” has a slightly softer pedal but actually has MORE clamping force than the 1 1/32” or 1 1/16”. The larger bore masters have Less clamping force but a harder pedal.

 

[bc3j]

15/16 isn’t too small for the system at all. Yes 15/16” has a slightly softer pedal but actually has MORE clamping force than the 1 1/32” or 1 1/16”. The larger bore masters have Less clamping force but a harder pedal.

I think we’re talking about the same thing. I look for the master and my brake system to develop 1200psi at the front calipers. Both can do it as I’ve used both in my car. The 15/16” pedal travel to achieve it is too long for my comfort level and the pedal effort does increase when I really push on the pedal to get to 1200psi. The 1 1/32” has half the travel and slightly more effort. It’s fine for me, but pedal feel is entirely personal for each driver. Just thought this might be helpful for someone.

 

[diesel_lv]

I just spoke with him 2 days ago and he said 2 psi is for front and 10 psi is rear. (The 2 psi valve is mostly for hot rods where the master is below the floor and the inlets of the caliper lines.) You may want to email and ask or phone and ask him again.. see the link for some facts and clarification...

https://www.ecihotrodbrakes.com/brake_facts.html

You are correct in the 10lb. I just went through my emails w him. So that is what is in my car. I purchased online the next day and had my brake shop install. For some reason I thought it was 2lb.

 

[Red63440]

Anyone notice anything wrong in this photo?

Yes I mentioned that I bought the complete rear drum rebuild kit including the wheel cylinders from doctor diff
View attachment 1105001

 

[Fran Blacker]

Anyone notice anything wrong in this photo?

Hard to bleed the brakes with wheel cylinder-s cockeyed. Wonder if he solved his problem?

 

[Red63440]

Look at the pawl for his self adjuster…was that just a temporary thing?

 

[Fran Blacker]

There are problems with self adjusters. but why is the BP cocked. This thread died with no resolution I guess?

 

[Mike67]

Yes residual valves are only needed when the mc is lower or at the same level as the caliper. With the mc on the fire wall there is always some "head " pressure on the lines.

 

[1STMP]

Yes residual valves are only needed when the mc is lower or at the same level as the caliper. With the mc on the fire wall there is always some "head " pressure on the lines.

Not in every case.
When the front pistons retract the fluid in the
front lines is directed back to the MC. A 2lb
residual check valve IS required in the front
circuit to keep this from occurring. A test
for their requirement is if you have to step
on the pedal twice to build pedal pressure.
On a 4 disc system a 2lb is required for each
circuit.
I have installed such on the brake system of
my truck with an incorporated proportioning
valve. The system works as it should.

 

[Mike67]

Not in every case.
When the front pistons retract the fluid in the
front lines is directed back to the MC. A 2lb
residual check valve IS required in the front
circuit to keep this from occurring. A test
for their requirement is if you have to step
on the pedal twice to build pedal pressure.
On a 4 disc system a 2lb is required for each
circuit.
I have installed such on the brake system of
my truck with an incorporated proportioning
valve. The system works as it should.
View attachment 1225272

Im not saying it will hurt anything and i stalled them on my wifes car hoping it would correct some issues, it didnt. Everyone's directions that I've read state that they are for a MC that is at/below the calipers. I think Stranges site mention the 2lb with drums.