Lame Braking

[Gyratingyak]

I just did a switch to manual 11.75” disc up front and my original 10x2.5 rear drums with a right stuff detailing 15/16” bore master. The pedal is rather mushy and isn’t confidence inspiring in the least. I ran all new brake line and used DOT 4 fluid. I’ve rebled the brakes and got no air out, I can’t get the fronts to lock up in a hard braking situation and the rears seem to be doing much less since the conversion. Do I need a residual pressure valve? Or do I need a different distribution block for front disc cars? My pedal ratio was approximately 5.8:1. Any help is appreciated, thank you in advance.

 

[rlem1977]

What is the year and car? Did you buy a Right Stuff kit or just buy parts? Did you bleed the MC prior to installing? You do have a proportioning valve installed?

I would start by re-bleeding the MC. Then re-bleed each corner - RR, LR, RF, LF. Depending on caliper/hose orientation, air can sometimes stay trapped and needs a little help getting out.

 

[Gyratingyak]

What is the year and car? Did you buy a Right Stuff kit or just buy parts? Did you bleed the MC prior to installing? You do have a proportioning valve installed?

I would start by re-bleeding the MC. Then re-bleed each corner - RR, LR, RF, LF. Depending on caliper/hose orientation, air can sometimes stay trapped and needs a little help getting out.

Right stuff MC but a Dr.Diff 11.75 disc kit. Maybe it does need rebled, I hope. Its just a little disheartening when I’m short on time to trouble shoot. I also don’t have a prop valve installed, only my distribution block

 

[Kern Dog]

On a front disc, rear drum system you need a proportioning valve. Not the case with a 4 wheel drum or a 4 wheel disc setup.
The pedal should be firm though with about 1 inch to 1 1/2" before firming up.

 

[Gyratingyak]

On a front disc, rear drum system you need a proportioning valve. Not the case with a 4 wheel drum or a 4 wheel disc setup.
The pedal should be firm though with about 1 inch to 1 1/2" before firming up.

Could I use my drum distribution block and pipe in my adjustable prop valve? Or do I need a disc/drum block and my adjustable prop valve? It also firms up near the floor, pretty far down, I thought I bled it relatively well, I guess I need to rebleed it again

 

[Yatzee]

I just did a switch to manual 11.75” disc up front and my original 10x2.5 rear drums with a right stuff detailing 15/16” bore master. The pedal is rather mushy and isn’t confidence inspiring in the least. I ran all new brake line and used DOT 4 fluid. I’ve rebled the brakes and got no air out, I can’t get the fronts to lock up in a hard braking situation and the rears seem to be doing much less since the conversion. Do I need a residual pressure valve? Or do I need a different distribution block for front disc cars? My pedal ratio was approximately 5.8:1. Any help is appreciated, thank you in advance.

Assuming that there is no air in the system I would think you're not generating enough force on the front pads or the brake pads themselves are garbage. I would start by inserting a manual prop valve in the rear circuit up near the master. This will allow proper braking bias front to rear. Shut off the rear circuit completely, bleed the front brakes, then re-bleed the rears. Now shut off the rears and take it for a careful test drive. With all the pressure going to the front only, you should easily be able to lock them up. If not, you're either not developing enough pressure, not displacing enough fluid for the front pistons or your friction pads are trash. You'll know if your short on fluid displacement by how far the pedal arm drops once engagement is made. The pedal should be near "rock hard". Try semi-metalics and try this test again. Don't forget that when all is said and done to re-bias the front to rear braking via the prop valve. Your pedal ratio is good but you may have issues with the size master on that system. 7/8 bore will firm things up providing you can displace enough fluid for the entire system without bottoming the master's piston. It is adviseable to have a single residual valve just after the manual prop valve for the rear circuit and adjust the shoes as close to the drum surface as possible without too much drag on the wheels. Once the shoes and pads are seated properly, further adjustments are required.

 

[Kern Dog]

It seems simpler to me with fewer connections to use a disc/drum proportioning valve but if you want to use an inline adjustable one to the rear, that makes sense too.

 

[beanhead]

Could I use my drum distribution block and pipe in my adjustable prop valve? Or do I need a disc/drum block and my adjustable prop valve? It also firms up near the floor, pretty far down, I thought I bled it relatively well, I guess I need to rebleed it again

Make sure your drums are properly adjusted...if they're not tight enough it will throw the whole system out of whack, and gradually become unsafe. I have a manual front disk/rear drum also and kept my stock distribution block, for my year (68) it is actually a safety switch and is what activates your brake warning light in case of fluid loss. It's also the 'tee' point for the one line from your master cylinder to the left and right front brakes. (The adjustable valve came with my kit so I used it) The proportioning valve is plumbed next to that block, in the line that runs to the rear. You need both...(or replace everything and use the factory disc/drum combo block that Kern alluded to) IF you absolutely can't get it figured out, give dr.diff a holler and talk to Cass..

 

[Gyratingyak]

I think I’m going to purchase the disc/drum distribution block from doctor diff, he recommended it. Then rebleed the brakes and see where that gets me. He said the 73 a body disc/drum block is a direct swap. Hopefully its as easy as that. The pedal feels sloppy, doesn’t get hard until its very far in its travel, its got me down because I keep running into problems with my swap and my front end rebuild

 

[khryslerkid]

Did you bench bleed the mastercylinder? Bleeding it in the car won't let the piston go far enough. Doing it on the bench, you push the piston all the way until it bottoms out.

 

[Billccm]

You will never get the same rock hard pedal as you did with manual drums. The 15/16 master cylinder has a long travel that gets some getting used to, but stops really well.

I recommend the proportioning block from Dr Diff to improve your car's braking, but it probably won't improve your pedal feel.

 

[Gyratingyak]

You will never get the same rock hard pedal as you did with manual drums. The 15/16 master cylinder has a long travel that gets some getting used to, but stops really well.

I recommend the proportioning block from Dr Diff to improve your car's braking, but it probably won't improve your pedal feel.

Well I definitely think it needs bled again and since its a direct swap I’m going to order the disc/drum distribution block from dr.diff. The pedal is way to spongey

 

[Yatzee]

On a front disc, rear drum system you need a proportioning valve. Not the case with a 4 wheel drum or a 4 wheel disc setup.
The pedal should be firm though with about 1 inch to 1 1/2" before firming up.

What are you smoking these days? A disk/drum prop valve has four connections plus two extra brake lines, inline has only two connections.

 

[Kern Dog]

What are you smoking these days? A disk/drum prop valve has four connections plus two extra brake lines, inline has only two connections.

You may not have understood the statement. Maybe you are still all lovy dovy about the Royal wedding???
4 wheel drum cars AND 4 wheel disc cars achieve their front to rear proportioning by way of different wheel cylinder sizes in a drum system or different caliper bore sizes in a disc system. The larger bore delivers more force so the front end gets the bigger wheel cylinder or caliper.
Read that again.
A 4 wheel drum AND a 4 wheel disc system still uses a junction block that sits in between the master cylinder and all 4 wheels. That junction block, also known as a "distribution block" has no proportioning feature. It does not alter the pressure. The rear gets the same pressure as the front.
On a front disc/rear drum setup, the dynamics of the two different styles of brakes necessitates the need for the proportioning valve. For instance, on a 73 Duster with non power disc/drum, the master cylinder is a 15/16" bore. The front calipers are 2.6 inches. The rear drum wheel cylinders are 7/8".
The function of the proportioning valve is to limit pressure to the rear brakes to avoid locking them up and causing a spin out.

 

[Kern Dog]

I think I’m going to purchase the disc/drum distribution block from doctor diff, he recommended it.

I get your point but to be accurate, in a disc/drum system, that block is actually a proportioning valve.

 

[Gyratingyak]

I get your point but to be accurate, in a disc/drum system, that block is actually a proportioning valve.

So my drum/drum should be fine if I use the prop valve to tailor the braking to the rear? No need to spend $85 more? The MC is 15/16 I believe the front caliper bores are 2.75 and the rear wheel cylinders are 15/16

 

[NJRR]

I just did a front disc brake conversion on my 70 Satellite. I used the SSBC kit and the right stuff 15/16" MC. I plumbed in the adjustable prop valve right off the rear line out of the MC. I have a very hard pedal. I did bench bleed the MC. If you haven't done that, I would try that first. It's possible that you have to bed in your front pads also which can take a few hundred miles.
i can't get my rear brakes to lock up even with the prop valve opened up all the way. The car still stops much better though than when it was drum/drum. It's possible my rear brakes aren't adjusted properly and that's why they don't lock up. Although, some members have stated they run disc/drum with no prop valve and have no problems. Try bench and rebleeding first.

 

[Kern Dog]

So my drum/drum should be fine if I use the prop valve to tailor the braking to the rear? No need to spend $85 more? The MC is 15/16 I believe the front caliper bores are 2.75 and the rear wheel cylinders are 15/16

Yeah, I think that makes sense. I forget that there are often many ways to skin a cat. (Who the heck came up with THAT saying??)
What the drum/drum distribution block does is : It serves as a multi-port junction to direct brake fluid from the 2 ports in the master cylinder to the 4 ports, One for each wheel. It also features a low pressure switch to alert the driver. Again, the drum/drum block does not alter the pressure to the rear wheels like the disc/drum unit does. It has NO proportioning function.
Having stated that, an aftermarket adjustable proportioning valve with a single inlet and a single outlet can be placed between the drum/drum distribution block and the rear brakes. You can put the aftermarket Prop valve anywhere within the rear brake line but most people put it where they can easily reach the adjuster knob.

 

[miller]

Going to assume you have a disc/drum MC, not one for drum/drum. One pot bigger than the other.

 

[YY1]

I am 98% convinced that Mopar did the proportioning via the bore size in the calipers/wheel cylinders.

Since you've changed that, it may need revisited.

The distribution block was simply that (and a warning light trigger).

Also I've noticed some modern reman/new wheel cylinders have tiny (I mean TINY) fluid passages from the lines. Not sure why but that could contribute to the cylinder not expanding the pistons in a timely manner.