Disc Brake conversion - can’t get pedal pressure

[hunt2elk]

It was part of the kit I bought. It’s a cast iron 15/16 bore View attachment 1105014

Not that it matters, but I would bet that one has a residual valve in the front chamber.

 

[dadsbee]

This is the second one (replacement) and I mounted it to the car and then bled it using the pedal and then attached the lines and bled the system

Quite possibly the problem. I did the on the car bleed, spent hours trying to bleed the master and ended up having to lift the rear end of the car as air was trapped in the nose of the master.

 

[diesel_lv]

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.

 

[moparedtn]

Quite possibly the problem. I did the on the car bleed, spent hours trying to bleed the master and ended up having to lift the rear end of the car as air was trapped in the nose of the master.
View attachment 1105015

Oh man, that looks scary!

 

[Ironbuilt]

Just my 2 cents, but a bench bleed is always more effective than a pedal bleed for master cylinders.

 

[dadsbee]

Oh man, that looks scary!

 

[khryslerkid]

It was part of the kit I bought. It’s a cast iron 15/16 bore View attachment 1105014

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.

 

[Ironbuilt]

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.

Sounds like you've done this a time or two.

 

[moparedtn]

Here, let me help you with that response...
"Although the front lifting arms aren't being used here, I only used the lift far enough to slightly raise
the rear of the car without the rest of the car touching the lift anywhere else - or sliding off the rear
lifting arms."

 

[Ironbuilt]

Here, let me help you with that response...
"Although the front lifting arms aren't being used here, I only used the lift far enough to slightly raise
the rear of the car without the rest of the car touching the lift anywhere else - or sliding off the rear
lifting arms."

Mechanics do it all the time. As long as the other wheels are touching the ground, no harm, no foul.

 

[dadsbee]

^^^, ya'd think I was stupid or something! Lifting pads are also stuck against the front leaf hangers...

 

[moparedtn]

^^^, ya'd think I was stupid or something! Lifting pads are also stuck against the front leaf hangers...

All I said was that it LOOKED scary, hoss....

 

[Ironbuilt]

All I said was that it LOOKED scary, hoss....

"If you're scared, get a dog!!"

 

[khryslerkid]

"If you're scared, get a dog!!"

 

[moparedtn]

"If you're scared, get a dog!!"

Something looking scary and actually being scared ones' self are two different things entirely...

One thing dying a handful of times cures a fella of is ever being scared again - of anything.

 

[Ironbuilt]

Something looking scary and actually being scared ones' self are two different things entirely...

One thing dying a handful of times cures a fella of is ever being scared again - of anything.

Sounds good...HOSS!!

 

[Ironbuilt]

Something looking scary and actually being scared ones' self are two different things entirely...

One thing dying a handful of times cures a fella of is ever being scared again - of anything.

But scared is scared. Just saying.

 

[moparedtn]

Sounds good...HOSS!!

Roger that, Buford.

 

[65_Satellite]

Not that it matters, but I would bet that one has a residual valve in the front chamber.

And would that be an issue? Here are the two ports

 

[65_Satellite]

Just my 2 cents, but a bench bleed is always more effective than a pedal bleed for master cylinders.

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