The Stovebolt Site Forums The Shop Area Driveline Lack of stopping power

I have 1954 3600 pu with disc brake conversion and rear drums. I have either a 7" or 8" power booster from CPP with their dual master cylinder. I have had the unit on for two years and have never been happy with the stopping power. I have a good pedal with rear shoes adjusted up but cant get a tire to squeal. Any suggestions? Have boost available because when you start motor the pedal drops a little. I measured the booster across the whole front edge and it seems to be 8".
Thanks Sharon

Let me give some more info. I purchased the disc brake kit off Ebay about 18 months ago. Everything bolted up just fine. I don't know the rotor diameter, can measure it tomorrow. I can also measure up the pad size. I don't think I can put on any bigger pads because that was determined by the caliper bracket supplied. How about a dual diaphragm booster? Grabbing at straws here. I can contact the supplier because he is still selling them, to ask him from what car or truck the rotor and pads are from.
Sharon

Hy Sharon Danella, did your kit include a proportioning valve? Do you know what the bore of the master cylinder is?

No proportioning valve because there are drum rear brakes and CPP said it wasn't needed. I'll have to look under the car tomorrow to find the cyl. bore size.
Thanks for the reply Sharon

Sharon,

Is this an under-the-floor master cylinder setup? Do you have the stock 3600 rear drums with Huck brakes, or did you swap in a different axle?

What kind of vacuum reading are you getting at idle?

Have you tested the booster- is it adjusted and functioning properly?

How much pedal travel do you have?

Did you install residual pressure valves (RPV) in the system? If so, where, and what psi rating(s)? A lot of new master cylinders do not have the integral RPV's in them anymore.....

There are varying opinions regarding the necessity of RPV's, but I put two (front & rear) in my 3600 system with an under-the-floor master cylinder, and it works really well. I also put them in my '63 4 wheel drum system with a firewall mounted power master cylinder, and it made a noticeable difference.

Every "corvette" style MC that I have seen that is used in most of these conversions does not have any RPV's in them.

I'm going to try and follow this thread and see how it works for Sharon, as I'm having issues with my brakes also. I grafted the front clip from an 87' onto my 64', Disc up front with the 87' rear axle. My problem is when sitting, after having bled the brakes, the pedal is perfect. But after starting the truck, the pedal travel is way too much. Still have brakes, but it travels too far for comfort. Not sure if I have to change the rod on the master cylinder, or if the master cylinder itself is bad. Has me scratching my head. Hopefully we can both get things worked out.

There was another POST on this same topic a while back. Can’t seem to find it............but .........the concern was pedal travel and the answer was the stock pedal being a tall long stroke will never feel like a modern vehicle brake.

I too am following this as this winter will be doing a disc upgrade on 1/2t chassis.
I don’t follow no proportioning valve when installing disc in front and drums in the rear.

I installed a front suspension from an 83 GMC half ton under the frame of a 1950 Chevy pick up and used the rear axle from the same vehicle with drum brakes.

So had front disc and rear drums both from the 83 GMC. Then mounted an aftermarket power brake system with a dual outlet master cylinder under the floor in the stock location.

I did use proportioning valves for front and rear.

At first the pedal would almost hit the floor before it would stop the truck, and it would lock the tires up, but had to travel a long ways down before it would stop.

I finally found the adjustment between the master cylinder and booster was spaced way apart and after some adjustments we made the pedal didn't have to travel far and you had plenty of stopping power.

I don't think you need a proportioning valve with disc and drum setup from what I have read on the parts people websites. My pedal travel is minimal. On Monday I am going to call CPP and see if a dual diaphragm booster would be better. The truck weighs 4,000 lbs you know. Maybe I am expecting to much but I don't think so. I believe with the old shoe set-up, up front when we moved pulling two motorcycles the brakes were better. I have a good pedal no matter when I hit it. And I may hit it with a sledge hammer soon.
Thanks Sharon

It think the need of a proportioning valve, mentioned many times by others here, should be something to look into. The calipers require more pressure to operate properly than the wheel cylinders on the drums do. Sounds from you description that your front brakes are not doing much work. If you cannot lock up the brakes, they are not doing their full job!

I hope you don't drive this truck on the highway like this. You could be putting yourself and others in danger.

I stay on city streets at 45 or less.

Just a stupid question here. I am getting confused about how I routed the lines on the master. Haven't crawled under the truck this morning but I think I put the front line on the back of the master which is furthest from the mounting flange. It may have the largest reservoir. Should the front brakes go to the largest reservoir?

You'll need to refer to the CPP instructions to be sure. The largest reservoir is usually for the discs. Front/back reference can be confusing, because the underfloor MC is mounted backwards from how it would be mounted on the firewall... Also, I think that all underfloor systems are dual booster since they are smaller diameter (7-8").

I really think that you are dealing with a booster issue (adjustment nut between booster & MC/low idle vacuum/defective booster) or that you need RPV's in your system. I have 4 wheel discs on my 3600 with the stock pedal with a CPP kit and everything works well. If it was a proportioning valve issue, I think that the rear brakes would at least lock up...which, if I understand correctly, isn't happening..

I got under the truck this morning and there is a proportioning valve that came with CPP kit. The front brake line comes out the rear of the master and goes into the front of the valve and the goes to the front wheels. I hope this is correct. The master seems to have the same size reservoirs also.
Sharon

UPDATE: I apologize for misleading about the lack of a proportioning valve. Now let me tell you that I have dual carbs from Tom Langsdon. Runs very well and a great low and very smooth idle. I wanted to put a PCV system on the engine so I put a tee on the manifold and split the vacuum to the booster and the PCV valve. I just realized that this cuts down the available vacuum for the booster. I plugged the line to the pcv valve and I have a very steady 16" of vacuum. Took it for a ride and on the third hard stop with very aggressive truck tread tires I had the tires squealing and wanting to throw you into the dash. Shoulder belt harness of course. I guess I fixed my problem by eliminating a stupid mistake of splitting that vacuum for the booster. Now I will run the engine fumes back to the ground.

Many thanks to all that helped me with this problem, you guys are great.

Thanks Sharon

Maybe just run the PCV system off a different port on the manifold ??? Use a stand alone big hose big port for the brake booster.

That's good news... I have dual carbs from Langon with my booster setup as well, and I have a split vacuum line for PCV and booster, but no problems with that arrangement. Your idle vacuum is on the low side, though...so I'm guessing that maybe you have a non-stock camshaft in there? If you still have the vacuum powered wipers (I don't), and that system is leaking, I believe that that can cause a vacuum loss. Hopefully somebody more familiar with the stock wiper system and having it leak can speak up?

Anyway, the section of a different PCV (they are calibrated differently) valve might allow you to split the vacuum line for your brakes- the one that you have in there now might have too large of a calibrated leak.

Stock camshaft, elec wipers, 8,000 miles on rebuild runs very well, uses no oil, idles very smooth. I can't believe there is an engine problem.

Sharon

I might add that we live at 3,000 ft altitude which should affect vacuum readings, right?

Sharon

Ah, Yes, roughly 1 inch per thousand feet- that explains that. So, you'd be right in the realm of "healthy" considering your elevation.

A booster should give you a completely different feel. I would suggest removing the vacuum feed and seeing if it makes any difference. I am not aware of any factory setups with front disc, rear drums and no proportioning valve. The pressure required for the disc brakes is higher than that for the drum brakes.

I do have a proportioning valve. I misstated that I thought there was none under the master cylinder. I didn't have a flashlite the first time I crawled under the truck.
Sharon

There are many choices in PCV valves too large or too much flow.

http://devestechnet.com/Home/PCVInstall

Another check: Does the current vacuum hose and booster have the check valve in place ?

Yes

This is from my learning curve. I had help installing the front clip from an 87' model truck, and we had to make our own lines. Somehow...someone got the front and rear lines going to the proportioning valve switched, and that was what was causing my pedal travel to be so bad.
We had bled the brakes enough that my legs are built like a body builder, and one caliper had to be replaced after I broke off the EZ out. We had the booster off the truck and had been from one end of the truck to the other. The scary part is that I've been driving it as it was for over 3 years now. And brought the 38' home from Kansas on a car trailer without any problems other than the pedal travel.
Just taught me that you're never too old to learn something.
Tramp

All's fine now that I have a dedicated vacuum source for the booster. just ran the vent tube back down to the ground. worked for 50 years a couple of more shouldn't hurt.

Sharon