OK, a bit of background:

1) I'm an engineer. That means I probably think I know more than I really do, but I've got all these neat formulas I learned at some point and feel the need to use them. Generally, I'm a pretty capable mechanic, having hand-fabbed a couple of race cars and done plenty of basic stuff, but I may have overlooked something stupid....

2) Car is a '69 Camaro, originally had 4 wheel manual drums, converted to power drums when I first put it back together. Stopping power with the drums was GREAT, but they were impossible to modulate and I got sick of trying to guess which way the car would dart when I grabbed the brakes. Swapped out for a "stock-style" disc brake upgrade around 2000 (can't recall the vendor, maybe SSBC?). Swap included 1-1/8" master cyl, Adjustable prop valve, new booster, 11" front rotors and 2 15/16" bore single piston calipers. Pads are HAWK HPS

The problem --
Under light to moderate braking, the brakes are fine. Under heavy braking, I can lock up the wheels fine, but modulating the brakes at med-heavy to full lockup is waaay too difficult. Biggest complaint is that the decel force vs. pedal effort is not linear at all. At low levels of braking, things are pretty flat, but at high levels of braking, I need to push on the pedal MUCH harder to get a small incremental increase in decelleration. Also, when things start to feel weird, the pedal hardly moves at all (or at least it doesn't feel like it).

Things I've checked/changed:
-- brake pads -- swappd out for another set of HAWK pads, no real difference
-- bled brakes (multiple times) pedal is hard/firm with the car of and no PB booster
-- bench-bled M/C (figured why not and bench bled again -- no air)
-- booster check -- deplete vaccuum until pedal is hard. Hold pedal and start car, pedal drops (so I'm getting power assist).
-- tweaked prop valve (I can get the rears to lock up before the front or the fronts to lock before rear -- it works).
-- front brakes are on front port on M/C, rears are on rear port (prop valve on rear)

Wondering if anyone has any thoughts on what might cause this (or if this is just "normal" GM brakes - my friend's old S-10 did the same thing). At some point, I'm planning on swapping out my rear and putting on rear discs, if I need to move to an aftermarket (LS1) brake setup to make this go away, I want to make sure I know BEFORE I spend the $ doing the rear so I can match things up.

Things that might matter(?):
-- M/C is "old" style round corner. Wondering if I'm out of volume (bottoming out front piston) and am considering swapping for the newer square-corner M/C with the "big" front reservoir.
-- New booster? I would expect that the PB either works or it doesn't. Seems unlikely to have it stop assisting part way through travel.
-- 10lb residual pressure valve for rear drums? Not sure if my M/C has the built-in RPV or not. Wouldn't expect this to effect the front brakes though

Ideas welcome.

I'm no engineer, except via the school of hard knocks, but what you describe sounds pretty normal for a vacuum boosted disk / drum system, because:

The vacuum booster does it's job best at low to moderate pedal pressure, beyond that, it ain't helping much.

The rear drums are "self-actuating", and therefore they are very hard to modulate. The harder you ask them to work, the more the primary shoe "wedges" and the harder it is to modulate the back brakes. If fact, it is impossible without reducing the front brake pressure lower than you'd want to under tconditions requiring lots of braking power.

Solutions:
rear disks will make the entire system much easier to modulate by removing the self actuation inherent in drum brakes.

A hydroboost will overcome the limitations of a vacuum booster. The harder you press, the more boost you get.

A properly engineered manual system will do this too. My Wilwood system is incredibly easy to modulate. I have THREE INCHES of pedal between the point where it first starts to stop and the point where the wheels start to haze. Modulation is pretty easy with that much pedal to work with.

1st off!! My car NEVER, EVER darted OR was twitchy w/ my stock 4-wheel drums!

I figure your front suspension need's looking into here! And don't forget the rag-joint too!


That said, bigger dia. rear tires will take some of the hard to handle outta the rear drums, but they will also make it harder to stop FAST!!

I remember how my car used to bounce up and down, reverse "slapping" my slapper-bars as I would try to haul her down from say like 80 mph in a panic stop as I stabbed the brakes as hard as I could, darn near bending my steering wheel and downshifted through the gears until I was in 1st! I wanted to open my door and drag my foot too, it was that scarey!!

And btw, w/ the WIFE going Bull-sh-t sitting next to me all the while!!!!

And now, if my "pdqCBB", 13" and 11.75" disc setup doesn't flat-spot all 4 tires, it will before I get done w/ her!!

pdq67

Thanks for the thoughts. Didn't realize that the booster is most effective at lower pedal pressures (never quite understood how they really work to modulate boost, I get the vac on one side, atmos pres. on the other, but not how they change power assist versus ???).

As far as the rear drums go -- I get the auto-energizing effects of duo-servo drums, but I would think that if I dial out the rears with the prop valve, that I should be working almost entirely on the front discs, which should give me more linear (but crappier overall) braking. I still get the weird deal where the pedal gets a bit hard and brakes feel funky.

pdq -- front end has been rebuilt w/ poly bushings, guildstrand mod, new balljoints, tie rods, etc. Can't find my alignment sheet, but it was something like -0.6 deg camber, 4 deg castor, 1/16" toe in. I suspect most of my issues with the old drums being darty/twitchy was due to poor adjustment of the shoes and lack of patience there. Discs stop straight, just feel funky.

I've been pondering dumping the booster for manual brakes to clean things up and get rid of a bit of complexity. Maybe I'll pick up a 2nd gen manual brake disc/drum master (1" bore) and a manual pushrod and see what happens. Reasonably cheap and would rule out the booster and MC...

Wait a minute!!.....I think I know what the problem is....You say the car was an original drum brake car which was converted to power brakes....Was the booster push rod positioned in the lower hole on the brake pedal arm?

Thanks for all the info you provided, that really helps.
I assume your master cyl bore size is 1 1/8"?
Is it a disc brake master cyl or the original drum master cyl?

You should have good modulation if you have enough vacuum available. What is your vacuum at idle?

What version of Hawk pad do you have? We have the HP+ and they won't brake well the first stop, modulation is poor when they are cold.

What is your front tire height?
Check that pedal, power brake booster should connect to the lower hole.

Check your bumpstops for signs of bottoming out. If you hit the bumpstops under hard braking, the wheels will suddenly lock! Feels like the brakes are messed up but it's the bumpstops.
David

I can verify that if your drums are worn over-size, BUT not the same worn size, side-to-side, they can and will affect engagement, plus if they are ever so slightly egg-shaped, they will pulse!

I had to turn all 4 drums on my old '64 Ford Fairlane SW b/c of them being junkyard uneven worn replacement, egg-shaped drums! It turned out fine afterwards even w/ a worn frontend.

One thing I did do periodically was, I'd accel forward say 50', then hit the brakes HARD, then back up 50' and do the same at least three or four times to make sure I had my "star" wheels adjusted evenly! I got carried away one time and had to pull my rear drums and back them off, b/c I did it too many times so watch that!!

pdq67

Re-checked the PB pushrod and it's in the correct (lower) hole, but it did remind me of something. When I converted my car over to manual trans, I think the brake/clutch pedals I got were for a '68 (maybe 67?). I didn't think anything of it, but poking around a bit more, it looks like the 67/68 and 69 brake pedals are different. I need to dig up my old auto pedal and compare. The booster pushrod (to the pedal) was also a bit long and I ended up trimming it a bit and screwing the clevis down to make things fit. There is still clearance at "full release" though, so the pedal isn't binding/bottoming and preventing the MC from releasing fully. I wonder what the differences are between 67/68/69 brake pedals?

I've forgotten a bit, but I'm 99% sure the master cyl is the one that came with the kit. I know I measured and it is definitely 1-1/8" bore.

I haven't thrown a vac gauge on it, but it's just a 307 with a peformer 204/214 cam. Running 18 deg initial + manifold vac for advance, so I'm fairly confident it's got loads of vaccuum. at idle.

Pads are the Hawk HPS (street) pads -- I had run a "race" (HP+) pad on my subaru and got sick of having poor cold stopping and high dusting.

Tyres are 245/50R16 -- 25.7" tall tyre F&R. Pretty close to stock as far as I can tell.

I'll have a look at my bumpstops, but I don't think I'm bottoming. Running Hotchkis springs F&R with a spring rubber up front to get some ride height back (top of tyre even with fender lip). Also running shorter poly bumpstops.

I'm guessing that either the master cyl is running out of volume (?) which is causing the pedal to get harder (but then I don't get why I can push a bit harder and still lock up?)
OR
the booster is flaky.

When I get a chance, I think I'll just pick up a C3 vette manual master cyl and go to manual brakes and see what happens. I don't mind more pedal travel if I can modulate...

Thanks!

check your booster vac fittings and hose. A soft hose can suck shut and shut off flow under high engine vacuum which is what you have with a manual trans and hard braking.

I had this happen on a flat bed truck, no fun!
David

Heh, in the "it's a small world after all" column, I was trying to remember the title of my old brake book (walked off during a road-racing minivan build a couple years ago) when I found this review of "Brake Handbook" by Fred Puhn on Amazon. Small world :)

***** This is a great book, April 17, 2000
By David Pozzi (Salinas, ca USA) - See all my reviews
If you want to really understand how brake systems work. Improve your existing brakes, or convert from drum to disc. Read this book first. Fred Puhn conveys loads of info on how these systems work. From basic hydraulics, to how brake boosters work, calipers, pads, rotors, drums. The whole works!
http://www.amazon.com/Brake-Handbook-Fred-Puhn/dp/0895862328/ref=pd_bbs_sr_1/105-5534755-5349210?ie=UTF8&s=books&qid=1190861614&sr=1-1

I think this is the book I had (good book) but still can't remember though and now it's out of print...

I didn't think people actually read those reviews! :)
you can probaby find that book on Ebay.

Hey guys,,,,thanks for a good read!

I found my old auto brake pedal and compared to the manual brake pedal I have now and they are exactly the same other than the pedal pad, so I guess I really do have the brake/clutch pedal for the '69 (I'm still not sure why the 67/68 and 68 pedal assemblies are different). So I'm off to convert to manual brakes to rule out the master cyl and booster.

Went about making a manual pushrod out of a 3/8" fine thread grade 8 bolt and it fits fine. Pulled my booster off and noticed some fluid at the rear of my MC -- good thing I'm swapping that out. Anyhow, since I'm waiting for the 1" MC to come in (should be this weekend) I figured why not bolt the 1 1/8" MC to the firewall and see what happens...

So, with the "wrong" 1 1/8" MC and manual front discs, rear drums, the pedal is actually pretty nice and progressive as far as I can tell, just really darn hard ( I wouldn't want to really drive around like this too much and I wasn't comfortable getting some speed up to try to do lockup testing). Anyhow, so far so good. Waiting for my C3 vette manual disc master (1" bore) to see how things feel. I figure for $20 it's worth trying to see if it works...

I've got that old book too!

pdq67

Picked up the Fred Puhn book from ebay (definitely the book I had before and the single best book I've ever read on brakes). Now I can read up on what I can't remember...

1" vette master is now in; originally had a spongy pedal that was a little too easy to bottom out. Bled the crap out of it and tapped the front calipers with wood end of my hammer to knock loose any trapped bubbles and all seems well now. (Note -- HELP!/dorman speed bleeders work well when trying to bleed by yourself. You can also use the speed bleeders with a vac pump if you're having trouble with the stock bleeder screws leaking around the threads, but it seemed like just pumping worked better). Now I have what seems to be a nice progressive pedal, although I think I can still just barely bottom out the master if I push like he!! on the pedal. If it ever stops being cold and rainy in MI, I'll road test her....

I'm thinking that if this is still crappy, I may move to Wilwood GM III calipers up front with braided stainless lines and reinstall the booster (with the 1" master). Hope would be for a bit better pedal feel due to the smaller bore master and stiffer calipers, plus 10 lbs/corner weight savings.

Have someone stand on the pedal then release while you watch the calipers. If something is bent, the calipers will twist or bend as the brakes are applied. This uses up valuable travel and can make the brakes act spongy.

The GM III calipers have been tried and seem to work, but smaller bores mean you need to go even smaller with the master cyl to maintain an easy pedal.

I talked to a guy who had a manual brake corvette of around mid 70's age. He was going to switch to power brakes, but first tried Performance Friction pads on it. He cancelled his booster order because the brakes became easy to operate.
Pad type is important. David

Well, it seems the new master cyl and booster did the trick. It seemed to stop fine with just the manual 1" master, but pedal effort and pedal height bugged me a bit, so I figured I'd go back to power brakes. I then realized that the booster that originally came with my SSBC brake kit had a pedal rod that was waay too long -- even after I shortened it a bit, the pedal was up at a stock manual brake pedal height (didn't need the brake switch bracket). Picked up a "new" rebuilt booster/master from Autozone (Cardone 50-1128) bench bled and installed it and it's all better. I know my old master was a bit leaky and I think the geometry or something was whacked with the old booster.

Now I'm planning on installing the residual pressure valve in the outlet to the rear line using new valves and seats from a master cyl rebuild kit (still running rear drums). I figure that should help get rid of the small "dead zone" I have at the top of pedal travel (or at least help reduce it)... Not sure why NONE of the "rebuilt" master cyls seem to come with these things.

I have a master cyl and booster from what I believe is a very original 69 Camaro and it has no residual pressure valve in either port. It was a disc/drum car.

I have read that the drum seals were redesigned to not require a residual pressure valve at some point, but don't know when that would have taken effect on the production line.
David

Not sure why NONE of the "rebuilt" master cyls seem to come with these things.

If you're referring to the C3 Corvette master cylinder you mentioned earlier, they don't have RPV's, as they were all 4-wheel discs. :thumbsup: