C5 Brakes on my 68 Chevelle

[calvin]

Last year I installed a C5 setup on my Chevelle. Well the brakes suck.. and I'm sure it's something to do with the master or master/booster combo. Can anyone tell me what I need to do to make these things work properly?? lol

Thank you ! :)

[andrewb70]

Last year I installed a C5 setup on my Chevelle. Well the brakes suck.. and I'm sure it's something to do with the master or master/booster combo. Can anyone tell me what I need to do to make these things work properly?? lol

Thank you ! :)

How about you tell us more about your system? Manual or power? What size MC? What pads? Is there a combo or prop valve in the system? Define "suck." What are the symptoms?

Andrew

[calvin]

Power brakes, proportioning valve and stock MC and brake booster that I know of (new..but stock)

[andrewb70]

Power brakes, proportioning valve and stock MC and brake booster that I know of (new..but stock)

What is the bore of the MC? Does your engine make enough vacuum to operate the power booster?

Andrew

[calvin]

I don't know the bore. Would a picture of the MC help? I was hoping there was a simple answer with an all too common swap?

Thank you

[130fe]

The swap is common BUT you have to use the correct size bore Master cylinder-

manual 7/8 or 15/16"
Power 1"
Hydroboost 1 1/8"

If you don't you will run into problems like this.

[Apogee]

"Stock" power disc/drum master cylinder bore size would have been 1-1/8"...and that would be most likely why the brakes suck. Another reason could be low vacuum with the factory single-10.75" booster, low CoF pad compound, improperly bleed system, etc., or any combination thereof.

Tobin
KORE3

[badazz81z28]

What does the definition of "suck" mean? I run an OEM disc/disc MC 1-1/8" bore and mine will lock the brakes up at will.

[calvin]

..suck definition :) .. lol.. Brakes are properly bled, starting at furthest wheel. Problem is when you hit the brakes you have to push very hard to make them work (like stand on the pedal).. it feels nothing like a power brake system of a vette..let alone a chevette! lol. I don't think it's vacuum..the always act the same whether at high or low rpm. Booster holds vacuum..master cylinder is new... I'm not sure.. I was kinda thinking my booster might have been to small?? It's a 10" I believe

Feels like I have small blocks of wood for brake pads..lol

[badazz81z28]

..suck definition :) .. lol.. Brakes are properly bled, starting at furthest wheel. Problem is when you hit the brakes you have to push very hard to make them work (like stand on the pedal).. it feels nothing like a power brake system of a vette..let alone a chevette! lol. I don't think it's vacuum..the always act the same whether at high or low rpm. Booster holds vacuum..master cylinder is new... I'm not sure.. I was kinda thinking my booster might have been to small?? It's a 10" I believe

Feels like I have small blocks of wood for brake pads..lol

What happens when you pump the pedal with the car off, does it get extremely hard? I run a small booster too. Again Stock disc/disc booster from a 81 TA.

I don't think the size of the MC is problem.

[calvin]

Yes, as far as I remember very hard8785787857

[andrewb70]

If you really want to know what's going on, put a pressure gauge on the front caliper.

If I was a betting man, which I am, I bet that booster isn't doing squat for you.

Andrew

[Apogee]

I'm with Andrew on this one...I see what appears to be a dinky little dual-7" diaphragm booster and is that a slightly modified, forced induction engine in the background? Without actual numbers, we can only guess your vacuum levels at idle. You've indicated that they're consistent, however without a number, that could mean consistently good, consistently marginal or consistently terrible. The dual-7" boosters provide substantially less gain/boost than the factory single-10.75" unit, let alone a dual-8" or dual-9" which would be a step up in the right direction, and your MC is about 25% too large with respect to piston area assuming a correctly sized (and adequate vacuum levels of 14 inches Hg or more) booster.

In summary, your booster is too small and your MC is too large...and at this point, your vacuum level and pad CoF's are unknown.

Tobin
KORE3

[calvin]

vacuum is right around 14in. yes it's forced. but regardless of if the engine is at idle or not..this thing has the worst brakes ever. CoF has no relevance at this point as it's more an inadequate "applying" force that's the problem.. it feels like i'm trying to slow a train with calipers from a camaro at any speed..lol

[andrewb70]

vacuum is right around 14in. yes it's forced. but regardless of if the engine is at idle or not..this thing has the worst brakes ever. CoF has no relevance at this point as it's more an inadequate "applying" force that's the problem.. it feels like i'm trying to slow a train with calipers from a camaro at any speed..lol

As Tobin and I both suggested, your booster is inadequate and your MC bore is too big.

Andrew

[calvin]

...so back to my original question.. what pieces do I need to make this work properly?

[andrewb70]

...so back to my original question.. what pieces do I need to make this work properly?

If you want to stay with power, see post #13.

If you want to go manual, you can use the Wilwood 7/8" bore MC.

Andrew

[calvin]

so an 8 or 9" booster and which mc? Are these just parts I can pick up at the auto parts store or do you recommend a different outlet?

Thanks for the help

[andrewb70]

so an 8 or 9" booster and which mc? Are these just parts I can pick up at the auto parts store or do you recommend a different outlet?

Thanks for the help

Look….we can't be 100% sure what you need. We are simply doing some "armchair diagnosing" that will hopefully lead you in the right direction to fix your car. You need to double check everything yourself, because for all we know, there could be other problems in the system.

If you go with power, I'd go with a 1" bore MC (this is what Tobin meant when he said your current one is 25% too big).

Andrew

[Apogee]

If you can fit it, I would suggest that you upgrade your booster to a dual-9" diaphragm unit, available through numerous aftermarket sources to be compatible with a '68 A-body, including but not limited to DSE, Summit (Tuff Stuff), MBM, CPP, Right Stuff Detailing, etc. I would then downsize your MC to a 1" bore option, aluminum or cast iron doesn't really effect performance, so take your pick, just make sure that your booster output rod length and MC are compatible. I generally prefer to run aluminum master cylinders since they weigh substantially less and are less prone to corrosion issues. Your vacuum levels are adequate for vacuum accessories, but not great, so the larger booster should do a better job of taking advantage of what you do have available.

Pad CoF is the single variable that can make the largest difference in terms of the overall brake torque, so to say that it's irrelevant isn't really true IMHO. If your system is currently set up with low CoF pads (low dust/noise/rotor wear pads are generally low CoF), then that is contributing to your low brake torque situation. Swapping out a set of low-CoF pads with a friction coefficient Mu = .25 pads (EE) for some with Mu = .50 (GG) will instantly double your brake torque, no other single item can or will provide that same degree of adjustment. Given properly sized booster/MC, adequate caliper pressures should be achievable to even make low CoF pads work, but that doesn't make them any less important a part of the big picture.

Another alternative would be to swap the vacuum booster for a hydroboost unit and run the MC you have now. HB's make a lot of gain and would take the engine vacuum out of the equation entirely, assuming you have power steering on the car.

Tobin
KORE3

[Pete68]

I did the C5 swap on my 68 Camaro and used the C5 master and booster and pedal feel is perfect, firm and linear with lots of feedback. GOt it used on ebay for $150