New issue. Now the brakes.
 

Hey all, I have a 70 C10 I am finishing up restoration on and I have a possible brake issue. I upgraded the front brakes to disc and kept the rear drums. I did add a CPP booster, master and proportioning valve kit. They all came together for my application. The part number is 6772CT. So my issue is when the engine is off the brake pedal feels great. I bled the entire system and there are no air bubbles I can see. When I start the engine the pedal gets really soft and will practically push all the way to the floor. Do I have a booster issue maybe? I have not checked what kind of vacuum I have yet. I do have a very mild cam that has an only slightly noticeable lope. Nothing radical. Where should I start with troubleshooting? Thanks

Re: New issue. Now the brakes.
 

Did you bench bleed the master? It sounds like air in the system or the master has an issue.

Re: New issue. Now the brakes.
 

I may need to do that. I did not bench bleed the master. Probably a rookie mistake. If I bought a pressure bleeder would it get air out for me so I dont have to remove the master?

Re: New issue. Now the brakes.
 

Yes, a pressure bleeder (which most people don't have) will bleed your system. The brake pedal should sink when you start the engine, as vacuum is applied. The amount of sink depends on whether your system is bled properly to begin with, though, and also depends on adjustment of the brakes; especially true if the drum brake shoes are not adjusted properly.

Re: New issue. Now the brakes.
 

It was my understanding that the drum rears are self adjusting. Is that not the case? I have not actually driven the truck yet and have been told they self adjust when going in reverse and stopping. Maybe I need to do that also.

Re: New issue. Now the brakes.
 

Yes, drum brakes (if equipped with adjusters) self-adjust. But if they are not adjusted enough after part replacement, the pedal will sink quite a bit. If you have replaced the shoes and done other work, the adjustment might be enough (if not done) to give a fair amount of travel- a really low pedal.

Re: New issue. Now the brakes.
 

just some food for thought, I recently picked up a 72 C30 with factory power disc/drum setup with a faulty master cylinder. Replaced the MC, blead the system and everything was great for about 5 miles, then my pedal went to the floor JUST as I was parking the truck at my house. I assumed I lost a hose or wheel cylinder. Nope... When I parked and turned the car off, as soon as the booster was evacuated of vacuum (pumped brakes when car was off) the pedal got perfectly firm, and would even stop the car when rolling with engine off. as soon as the truck was on, the booster would assist me in pushing the pedal strait to the floor every time. Brake fault dash light would also turn on. (Happy to see it worked!)

spent two days bleeding every corner and at the MC and Prop valve. no air, clean fluid.

Not seeing many options, I bought a second new (reman) master cylinder. was VERY careful to ensure thorough bench bleeding (20 minutes at least, I was to be CERTAIN it was clear of air!) I put it back on the truck, was able to actually only bleed it at the MC and the Prop Valve, and brakes were prefect.

I am not saying your issue is EXACTLY the same as mine, as my issue was with stock (rebuilt) components, but if you have a firm pedal without your booster helping, and when your engine is on the booster helps push the pedal to the floor it could very well be the MC (air or fault). I think in my case my MC was failing internally and allowing fluid to pass between the internal seals, but I could see air causing a similar problem.

If your rear brake shoes are properly adjusted, and your confident there are no leaks in the system anywhere I would look at the Master Cylinder. Generally, master cylinders are only two nuts and two hoses, I would start by taking it off the truck, putting it in a vice with a master cylinder bleeding kit (click here for link if your MC didn't come with one) and spend some time carefully bleeding it until your comfortable there is no air in it. Its possible to bleed it in the truck but its way easier to control the 3/4"-1" slow piston pushes when its not in the truck.

If you can blead it totally free of air, and your still having issues... id make sure your drum shoes are adjusted and then start thinking about if you have a faulty master?

Booster shouldn't be your issue, it helps push, it seems to be doing that just fine.

Re: New issue. Now the brakes.
 

Thank you very much. I appreciate the time you took to post. Unfortunately my truck is at my parents 30 minutes away and with my work schedule I will not be able to get back to it until Friday. I plan to bleed the MC, re bleed the brakes and make sure the rear drums are adjusted properly.

Re: New issue. Now the brakes.
 

If you didn’t bench bleed the master that is most definitely the problem
Mark

Re: New issue. Now the brakes.
 

So I have an update. I worked on the truck today and did the following. First I checked the rear drums to see if the needed to be adjusted. They were way out of wack so I adjusted them until they just began to drag. Tried the brakes and no difference. I removed the master cylinder and did a bench bleed. Installed it and re bled the entire brake system. Didn't help any so I drove the truck down the driveway so I could back up and try the self adjustment on the drums. Made no difference. Pedal goes all the way to the floor. The rear brakes will lock up on gravel but only after being fully pushed to the floor. I almost feels like the front brakes are doing nothing at all. I put it back in the shop and did another bleed. Didn't see any air. So I said screw it and took it for her maiden voyage anyway. The roads were we are are not busy so I went. The truck rides great and felt so good to drive it again. I was able to stop obviously but I had to plan way in advance. I drove it less than a mile. Also my brake warning light comes on and stays on most of the time. Could it be my proportioning valve? Very frustrated because this is what is keeping me from getting an inspection which would complete my resto.

Re: New issue. Now the brakes.
 

Did you change the pedal ratio to 4.5 to 1?
Power is like that.
Drum is 6 to 1.
Usually a new connection hole on the pedal about 1 1/2 inches lower gets you close.

Sometimes changing the ratio works, sometimes not.

Re: New issue. Now the brakes.
 

That is new info to me. Nothing in the instructions from CPP said anything about doing that. I have their disc brakes up front and original rebuilt drum in the back.

Re: New issue. Now the brakes.
 

Some info here.



https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

Re: New issue. Now the brakes.
 

So I just did another master cylinder bench bleed and bled it for a solid hour to make sure all air was out. Reinstalled it and bled the brakes. No change in my issue. In starting to get really pissed and don't know where to go from here. Sucks!

Re: New issue. Now the brakes.
 

I sent you a PM.. I couldn't find that part number listed on the CPP site.. Is this what you have?

https://www.classicperform.com/Store...ck/6772BBD.htm

If this is the booster/MC you have, you don't need to change the pedal ratio.. The linkage to do that is included with the conversion mounting bracket..

Re: New issue. Now the brakes.
 

Yes sir I believe that is the one I have.

Re: New issue. Now the brakes.
 

Also I did check the vacuum at the carb and I only have 13 inhg. Maybe I need a vacuum boost pump. I assume my cam I the reason for the low vacuum.

Re: New issue. Now the brakes.
 

While your cam might be a factor, your primary throttle opening to sustain an idle can be the real cause of marginal vacuum to use your power brakes.


What’s your idle rpm?
Where’s your vac advance plugged onto?

What’s your initial timing?

You need to close the primary as much as possible.

Re: New issue. Now the brakes.
 

If possible, post a picture of the actual booster/MC..

About 3 years ago, I completely redid the brakes on my '71.. I also converted to power brakes by using a "conversion kit" from Summit.. Their booster is a dual diaphragm design.. A dual diaphragm works better with an engine that produces low vacuum.. MY BBC has about 12 - 13 inches vacuum at Idle and my brakes are terrific...

https://www.summitracing.com/parts/sum-760197

Re: New issue. Now the brakes.
 

Here are some pics from when I mocked it up to make sure everything fit right.

Re: New issue. Now the brakes.
 

That's a "stock type" single diaphragm booster.. Not my first choice for a modified engine with lower than normal manifold vacuum, but will probably be ok for your "mild cam" application.. But the booster is not your problem... If the booster was defective or not compatible with your engine, you'd have a hard pedal, not a soft one that sinks to the floor... That symptom indicates a leak in the hydraulic system, air in the system or a defective master cylinder..

Concerning the rear drum brakes: They are self adjusting and will maintain proper adjustment as you put miles on the new shoes.. BUT-- You have to give them a good "starting point".. The way to do that is to tighten the shoes up so that you can't turn the wheel by hand.. Give the adjuster 3 - 4 more "clicks".. Then loosen the adjuster until you can just rotate the wheel.. You'd think the shoes are still too tight but it's the newness in the shoes coupled with the freshly machined drum surface.. After applying the brakes a few times while driving, the shoes take a set to the drums and will have the proper clearance...

Re: New issue. Now the brakes.
 

So I may have just found a major issue. I looks like I installed my calipers on the wrong sides. My bleeder is below where the hose comes in. I bet my front calipers are completely full of air.

Re: New issue. Now the brakes.
 

Yep... Bleeders have to be at the highest point....

Re: New issue. Now the brakes.
 

I will swap them tomorrow evening and see what happens. Fingers crossed that this will fix my entire problem.

Re: New issue. Now the brakes.
 

I bet that will fix it....