Hi guys, it´s me again!haha Well,I´ve been searching for ideas for a while now, and did several searches on the Hamb too. But I still haven´t got found what could be wrong on my car... So first off, I have a Mc Gauthy´s power disc brake conversion on my 56 Chevy. All new parts, calipers ,hoses booster, mc,lines, dropped spindles,you name it... It brakes well so far, but in an emergency it would lock up the rear tires before the fronts, sending you on a real wild ride. I couldn´t make the fronts lock up at all! Almost had to change pants when that happened for the first time,hahahaha! Here´s what I tried until now: -I´ve a manually operated wilwood pressure valve installed in the rear line.It´s supposed to increase and decrease braking power to the rear. It came with the kit. -I benchbled the mc a couple of times, I´ve got a good pedal feeling though. -I checked if the mc drags, but no, it even has very little lash, maybe .001" -I redrilled the brake pedal 1" lower for the rod clevis bolt, just like the instruction sheet said. There are no visible leaks at all, and the reservoir is full all the time. Could the MC be bad from new? Thanks for any input, Chris
A mentioned above you will need to have a residual valve inline. I also had the same problem on my 56. The issue was that with the BB and lowered front end there was not enough weight over the rear axle under braking. I went from a Wilwood valve to a standard Chevy one and that cured the problem in all but the very hardest of braking. On the suggestion of another tri chevy owner I fitted a front anit roll bar which cured the problem completely. Not sure but something to do with the bar reducuing the amount of weight transferance to the front wheels and so leaving more weight over the rears. What tyres are you running on the rears? If they are MT sportsmans then that will also cause a problem as they have so little grip that under braking they will just skid over the road surface rather than slow you down. Not sure if this helps or not.... P.
Thanks!That are good ideas. I thought a 2psi residual valve is only needed in applications where the mc is at a lower or equal height as the wheel cylinders, to avoid the drain back and keep the brake shoes close to the drums? Maybe I´m wrong I only have a 327 in it and the car is lowered equally ( airbags), tires are 225/70-15 Cokers. The front actually did lock up with the old stock drums, so I can´t believe the new power discs have worse braking abilities than the drum had...? It feels more like the braking power at the rear is fine, while the front is by far not good enough.
The redesual pressure valve prevents the springs in the drum brake from compressing the wheel cylinder so much that normal pedal strokes will not move the shoes into contact with the drum. This valve is usually incorparated in the MC. Of course it must be a disc/drum type of MC. You also need a proportional valve, yours is adjustable. The problem you may be experencing is un-equal brake balance. The front brakes are working as they should, but when you "panic stop" the front nose dives decreasing the weight on the rear and causing the brakes to lock. This problem might be corrected by adjusting the proportional valve. Then again, it just might be something that can't be fixed due to the physics of the car. Remember, when you apply the brakes the weight of the car shifts to the front. The disc brakes can take the load, but at the same time the rear is getting lighter. Just cause the don't "lock up" doesn't mean they are malfunctioning.
Did a disk front conversion in the wife`s 66 mustang, and that locked up in the rear, asked around, and was told by a few folks that had done the conversion, to put smaller wheel cylinders in the rear, ( 7/8 ) in this case. and it`s stopping like a new one. something about keeping that 60% - 40% thing happening. worked for me. Martin.
It doesn´t feel like the car was nose diving due to weight tranfer, and it also happens with a full trunk and p***engers in the back seat. But when fully loaded ,stopping the Bel Air is quite a challenge with the discs now. I wasn´t with the drums. As I stated above, I don´t think the front brakes work ok, something must be wrong there. Would this proportioning valve help some? Here´s the car
Adjust the proportioning valve to reduce pressure to the rear brakes. This will allow the front brakes to work harder before the rears lock up.
Ok , now it´s getting tough . I ´ve changed the master cyl.( pwr discs front and stock drum rear) for a new one and added a proportioning valve... and guess what... It still overbrakes and locks up the rearend. I was rainy today and it´s not fun, I can tell ya. Anyone´s got an idea what I could try next? Thanks for your help! Chris
This may sound stupid, but I'll suggest it anyway..... Are the rear brakes adjusted properly??? I used all stock chevy parts on my 65 swb pick-up: Chevelle discs, master cylinder/booster/proportioning valve and 12 bolt rear axle. NOT all from the same car, but all from the same vintage... late 60-early 70's. Truck sits low at a slight rake. 205's front and 255's rear. Had the same problem with back brake lock up. I added a front sway bar and that helped a lot but didn't cure the problem. I finally backed off the adjustment in SMALL increments with a test drive in between each adjustment til I got it to stop hard without locking the rears. I've driven it like this for 17 years in all kinds of weather at all kinds of speeds (hi-way/city, etc), including rain, snow and ice and have had no problems. I got it dialed in so that I can jam the brakes on under emergancy conditions (on dry pavement) and come to a full stop quickly and safely with out locking up the rears or taking a nose dive. I had the same problem with my 55 Chevy wagon and solved it the same way. It had Heidts brakes and spindles with the stock 55 rear brakes. Sat the same as the truck with 205's front and 235's rear. Please understand.... I'm not suggesting that you disable the back brakes; that isn't what I did and I'd never recommend it... My back brakes will lock up, on ice or loose gravel, for example, just as the fronts will lock up. I'm suggesting because it looks like you've done everything else and sometimes, it's the simple things that get overlooked. Rich
yeah..the MC are sometimes bad from new.. had to replace one after about 300 miles that went bad..front brakes would lock up and stay locked until I rebled the front calipers.
I think swapping out the M/C might be a good idea, you could have gotten a bad one and make sure it's for a disk front/ drum rear car... I'd also pull the rear drums and inspect your pads, slaves, and axle seals. Just a little bit of break fluid or oil on the pads will cause them to lock up the way you're describing.
The brake lines aren´t copper, they are called CuNiFer, some kind of copper/ nickel/ iron mix which is rustfree and good to work with. It doesn´t feel like the rear brakes to much, the front doesn´t get enough power. Maybe it has something to do with the master cylinder to caliper ratio. MC is 1" and calipers are midsize GM, like Malibu and the like.
"bar reducuing the amount of weight transferance to the front wheels" Only when cornering. In a straight line any bar is un-loaded and does nothing. I think all your bias/proportion stuff may be working, but is out of range with your existing rear stuff (i.e., not enough to prevent locking). Obvious cure: look up the part number of your rear wheel cylinders. Look at an interchange book for an identical mount (probably GM, similar year but smaller brake or narrower lining like 6 cyl.) but with the next size (1/16 or 1/8") smaller diameter cylinder.
I bought an Avanti once that had a Mustang 9 inch swapped under it. It also locked up the rears much too early even though I tried to back them off as much as a prop valve could. A smaller cyl diam for the rear wheels made the difference I needed. The original Mustang rear wheel cylinders were huge.
Thanks C9,I did, and I guess that´s only one of the problems. I am sure this would cure the overbraking issue but the car wouldn´t brake at all then. I had a manual , adjustable proportioning valve before I changed to the new master and new automatic proportiong valve. When I decreased the braking power to the rear , like I would using different rear brake cylinders, the pedal feel was identical but the car would hardly stop at all.I even bent the pushrod clevis once trying to stp that car... I take that as a hint that my rear brakes are ok, but the front brakes are too weak. The rear only locks up under had braking. But the front ought to lock up before the rears and that shows in my eyes that there´s not enough power to the front. What could be wrong with the front calipers? They are midsize GMs like they were used on 80s Malibus and 10,5" rotors... maybe the calipers are too small for the master?
You guys rule! Problem solved!!! It brakes better than dropping an anchor now! I loosened the adjusters at the rear drums about 15 clicks on each side, and viola :front locks up at 3/4 of the pedal travel and all 4 wheels at full travel. It also feels like there´s way more travel in the pedal than before . Thanks so muchfor your help guys!!! Chris
One further comment. The rear wheels should apply slightly before the front according to the factories and brake aftermarket people. There's a metering valve up front that delays the onset of front braking for a few fractions of a second. That said, most hot rods - including my 32 with Mustang 11" discs up front and Merc wagon 11" x 2 1/4" wide drums out back - don't run one of these dedicated metering valves and braking is quite good. It may be that the metering valve is for four wheel disc brake cars. A small fwiw - if you're running a Ford 9", you can get 11" drum brakes in 1 3/4", 2 1/4" and 2 1/2" widths. Do a little thinking about what you want and what you need. A little less power in the rear brakes may balance out better than the bigger drum brakes. For that reason I chose the 11" x 1 3/4" drum brakes for the rear of my 31 on 32 rails roadster. It will be lighter than the 32's 2400# with top and roll bar - 2300# without - and looks like the 31 will come in just about 2200#.
Ok, here´s another update. Maybe somebody will find this helpful in the future so I´m gonna post it. By backing up the brake shoe adjuster the overbraking problem on the rear drum got better, although on some occasions they still seem to lock up a tad earlier than the front discs. I was at a buddies garage this morning and we had the car on the brake power tester (I don´t know the correct word for that thing, it looks like a ch***is dyno , the wheels are on spools and it measures braking power in Kilo Newton. The front discs had 2,6 kN left and right The rear drums had 1,5 kN also left and right. Looks good so far. Since the rear drum brakes look like they have been rebuilt some time ago, I figure they could have aswell been turned out. Would a drum ,that had been turned out too far ,cause the shoes to wedge themselves up inside the drum , causing the drum to lock up? I´ll got out in the garage now and measure the drums... Chris.
When you are bleeding the front calipers are the bleeder valves on the top of the calipers? If not you may have air trapped in the calipers. If the bleeder is not on the top take the calipers off of the mounting brackets and put a wood block in between the brake pads turn them so the bleeder valve is on top and bleed the brakes. I had to do this on an aftermarket disc brake kit with GM midsize calipers....
Drums turned past the design limit can cause all kinds of problems such as poor friction contact , wedging or over heating which can cause drum warpage leading to pulsation when braking . I even had a fun [not really] experience once with a front drum which had been turned too much . I was slowing for a stoplight when the outer and inner portions broke away from each other - think a ring and a plate - which yanked the steering wheel hard toward on-coming traffic . Luckily the vehicle was already moving relatively slowly so I was able to correct the direction before any damage was done , except my pants .
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