MBM Disc Conversion Kit?

[Chewievette]

I wont get into all the little details but I purchased MBM's front disc conversion kit (1968 c-10 factory power brakes) this past spring along with a stainless lines kit from LMC. Installation started in June and was completed within two weekends. I had a problem initially with not being able to get a firm pedal which was eventually tracked down to a bad proportioning valve. During the post install bleeding of the new prop the pedal to booster rod bent forcing a further delay and numerous problems getting parts (not from MBM) for a new rod. That was finally solved and I installed the new rod this week and re-bled the entire system. I finally have brakes again! During normal around town driving the brakes feel fine and normal but when sitting at a light I can push the pedal all the way to the floor, I've never been able to do that in another vehicle. Also during a panic stop it takes a considerable amount of force to lock up the wheels, much more than I would expect, the pedal needs to be firmly to the floor to lock up. I have bled the system several times now and cannot find any additional air. While the brakes are functional they just dont feel right. Any suggestions why I'm not getting a good pedal feel are most welcome. I have tried everything I can think of and I've read MBM's instructions and troubleshooting manuals cover to cover several times.

[lolife99]

Who is MBM?

[Chewievette]

They are... apparently not as common as I was led to believe.

http://www.mbmbrakeboosters.com/

This is the kit that I have. I also bought their 8" dual diaphragm booster, master cyl, and prop valve kit
http://www.mbmbrakeboosters.com/inde...emart&Itemid=6

[dwcsr]

If your pedal goes to the floor with lite pedal pressure the master is bad. When you get the replacement and bench bleed it do not do full strokes with the piston. Push it in only about an inch and then release and repeat til its bled. On some GM masters going a full stroke when bleeding damages the inner cup. If I remember MBM uses that type of master in their kit.

[Chewievette]

when i block off the fluid outlet from the master I get a very firm pedal. all sources tell me that indicates a good master. will it still test good if the inner cup is damaged?

[Chewievette]

Replaced the 1" master from the kit with a 1.125" master from AZ and rebled everything again being sure to bleed the master with partial strokes. It stops a little quicker, pedal feels normal during normal driving and braking. Pedal is still able to push all the way to the floor while stopped and harder than normal braking is still unable to lock the wheels no matter how hard you push. I dont know what else to try. I've been working on these brakes for the last two and a half months, everything in the entire system has been replaced at least once, I have bled the system more times than I can count and I have been back and forth with everyone I know that knows anything about brakes and I am completely out of ideas. Anything you guys can think of? I'll try it. Thanks

[dwcsr]

Is the pedal at the top against the stop when you don't have your foot on it?

What type brakes are on it, disc/disc or disc/drum drum /drum?

If rear drums how did you adjust the rears? I had to use a drum micrometer to get the shoes just right and resolve a low pedal.

Is the brake light connected and working and is if off when its connected?

How does the pedal react when you have the truck turned off and the vacuum bled off the booster. Does it still go to the floor with a slight pressure?

How far does the pedal travel engine off and how far engine on?

I'm thinking that your rears are not functioning properly if you can't lock them up.
It could be the proportioning valve has shifted and blocking fluid to the rear. I had a very low pedal and once I properly adjusted the shoes, centered the valve and bled it again, it all worked as it should with a good pedal.

[Chewievette]

Is the pedal at the top against the stop when you don't have your foot on it?
Yes

What type brakes are on it, disc/disc or disc/drum drum /drum?
Disc / Drum

If rear drums how did you adjust the rears? I had to use a drum micrometer to get the shoes just right and resolve a low pedal.
Rears adjusted to give a slight drag when the drum is on and turned

Is the brake light connected and working and is if off when its connected?
Yes and Yes

How does the pedal react when you have the truck turned off and the vacuum bled off the booster. Does it still go to the floor with a slight pressure?
How far does the pedal travel engine off and how far engine on?
Pedal can be pushed all the way from the stop to the floor with engine off as well as on. Engine off requires more pressure but its still easily possible.

I'm thinking that your rears are not functioning properly if you can't lock them up.
It could be the proportioning valve has shifted and blocking fluid to the rear. I had a very low pedal and once I properly adjusted the shoes, centered the valve and bled it again, it all worked as it should with a good pedal.
The only thing that is not "as it should be" is the parking brake has been disabled. When it stops raining and I regain some motivation I will go enable them, readjust the drums and try again.

[dwcsr]

I think the trouble is in the back. Connect up the e brake and buy this tool and adjust the rear brake with it then bleed the rears again. The rears have to be adjusted up properly before you bleed them.

K-D Tools KD 3377 Drum Brake Resetting Tool

I don't know why but I had the same issue and until I used this tool and made sure the prop valve was centered and re bled it.

Make sure the axle bearings are OK with no excessive play. Look for leaks and make sure the hoses are not ballooning when you apply the brakes and the shoes are on correctly. Thats about all I can think of that could be wrong

[Chewievette]

Thanks for the help! Seriously. I'll tear into the back tomorrow and let y'all know how it turns out.

[dwcsr]

Just a reminder that if the set has a short shoe the short shoe is on the front.

[Chewievette]

Reinstalled the parking brake, readjusted the drums, rebled the rears. No change.
I'm back to being stumped again.

[dwcsr]

Originally Posted by Chewievette

Reinstalled the parking brake, readjusted the drums, rebled the rears. No change.
I'm back to being stumped again.

If you apply the e brake how is the pedal?
I usually adjust brakes by adjusting them up until the wheel will not turn and then back off til it just turns freely then back off the other side the same amount.

is it s stock booster unit or an MBM?

[Chewievette]

I never thought to check the pedal with ebrake, I'll do that in the morning.

I'll try adjusting the drums again. I wasnt able to find one of those tools locally so I'm doing it the cheap way. At this rate I might as well convert the rear axle to disc then I'll have a 100% new brake system (of course at this point the only thing I bought with the truck are the backing plates and drums)

The booster is MBM's 8" dual diaphragm unit. If that turns out to be bad I'm going to be incredibly upset with MBM. I've already changed out their master and prop valve trying to fix this.

[Chewievette]

I just went out and played around with it for a minute, with the engine off the pedal is noticably firmer but on or off the pedal still can be pushed to the floor. ebrake application makes no difference.

Right now I'm looking at changing to an adjustable proportioning valve. and wondering if this might just be a weird characteristic of this style booster? It would be annoying to change either at this point but I dont really care anymore, I just want it to work.

[Chewievette]

Random thought, can I eliminate the prop valve from the system for testing? I obviously wouldnt leave it like that but if I run a hardline direct from the master to the front system and plug the rear to test the front. Then swap, hardline the rear and plug the front. Will that get me anywhere? I'm trying to avoid spending another $70+ on a prop if this one is good. If I take it out of the system and the problem is still present then the prop is good.

[68 short step]

to eleminate where the problem is, which i suspect its in the rear. take the haadline loose from the flex line (on the rear), then cap off the hardline, press pedal a couple of times, it should get hard and not go to floor...if it continues to go to floor, do the same to the front pass side, then the driver side.. the soft line brake hoses may be expanding on ya.

[Chewievette]

Originally Posted by 68 short step

to eleminate where the problem is, which i suspect its in the rear. take the haadline loose from the flex line (on the rear), then cap off the hardline, press pedal a couple of times, it should get hard and not go to floor...if it continues to go to floor, do the same to the front pass side, then the driver side.. the soft line brake hoses may be expanding on ya.

I'll give it a try.

All of the lines, hard and soft are new. What would be the fix for this if its the problem? Braided stainless flex lines?

[dwcsr]

Originally Posted by 68 short step

to eleminate where the problem is, which i suspect its in the rear. take the haadline loose from the flex line (on the rear), then cap off the hardline, press pedal a couple of times, it should get hard and not go to floor...if it continues to go to floor, do the same to the front pass side, then the driver side.. the soft line brake hoses may be expanding on ya.

Thats the next logical step, start blocking off lines.

I just remembered someone here had his calipers on upside down and he had a similar pedal. Is the bleeder for the disc caliper on the top or bottom on yours. They should be up at the top.

[Chewievette]

Top.

I'll start blocking off lines when I get some time later today.

[Chewievette]

Blocked the rear brake line before the flex hose: no change
Blocked the front line out of the prop: hard pedal
Blocked the front lines before the flex hose: hard pedal

I'll have to go back out and block off one then the other to see which side its on and go from there. Its confirmed either the front hoses or calipers.

[Chewievette]

When both front lines are blocked the pedal is hard and stops approx 3" above the floor
When only one side is blocked the pedal is hard and stops approx 1" above the floor, this is the same for both sides.
I had my friend pump the pedal while I held the hose and there is some definite expansion going on but I have no idea how much is acceptable. I'll start shopping around for some better lines, probably stainless.

Is there any way to isolate the calipers from the hoses? I would like to test the hoses without the calipers attached.

[dwcsr]

Originally Posted by Chewievette

When both front lines are blocked the pedal is hard and stops approx 3" above the floor
When only one side is blocked the pedal is hard and stops approx 1" above the floor, this is the same for both sides.
I had my friend pump the pedal while I held the hose and there is some definite expansion going on but I have no idea how much is acceptable. I'll start shopping around for some better lines, probably stainless.

Is there any way to isolate the calipers from the hoses? I would like to test the hoses without the calipers attached.

Take a c clamp and clamp the piston all the way in the caliper. That way you have very little fluid and hopefully no air. I don't know that you should see the expansion of the lines but being rubber I suppose they almost have to little. I really don't know what would be acceptable. Are they new? if not change them to a brand name hose.

[Chewievette]

I changed the no name hoses out for a pair of Dorman's this afternoon and the pedal is Much better. Finally it definitely gets firm before it hits the floor. I drove the truck to work and back last night and I didnt have any problems except just after starting to come home the pedal goes to the floor the first push (change from park), second push immediately after gets a firm pedal. I dont know what that is all about... but again during a panic / hard brake situation it literally takes both feet and almost all the force I can apply to even give a hint of lockup. Maybe its just a quirk of these brakes that I'll have to live with? I dont know...

All I know is that I will NEVER recommend this brake kit to ANYONE.
Thanks for all the help and guidance! You really helped me keep some sanity during this long annoying process.

[dwcsr]

Originally Posted by Chewievette

I changed the no name hoses out for a pair of Dorman's this afternoon and the pedal is Much better. Finally it definitely gets firm before it hits the floor. I drove the truck to work and back last night and I didnt have any problems except just after starting to come home the pedal goes to the floor the first push (change from park), second push immediately after gets a firm pedal. I dont know what that is all about... but again during a panic / hard brake situation it literally takes both feet and almost all the force I can apply to even give a hint of lockup. Maybe its just a quirk of these brakes that I'll have to live with? I dont know...

All I know is that I will NEVER recommend this brake kit to ANYONE.
Thanks for all the help and guidance! You really helped me keep some sanity during this long annoying process.

What you describe about both feet to stop sounds like a booster problem. Or a low vacuum issue.

Tighten up the backs a little a and see if it gets better on the pumping issue.

In my 72 the pedal isn't exactly hard when running. It gets very firm when shut off and does go down a little when you start it and the booster does its assist. If its traveling more than 3 inches there is still a problem.

Again I had an MBM booster and had to change it about a year later with a stock rebuilt one. I purchased a booster , master and prop valve all of which I replaced with stock rebuilt units before a years time. We stopped sell their line because of the issues we had and them swearing no knowledge of any product issues. We didn't feel we could send this stuff out to the public being brake parts after the trouble we had in house. Good ole made in USA parts no better than Chicom parts