Weak Rear Brakes

[bobs66440]

I've been playing around with this for months now. I have GM front discs, S-10 rear with the clip in wheel cylinders (not positive of the year or the bore sizes), and the mid-70's Ford master cylinder (manual disc/drum) that was recommended and I thought is common to these cars. Wilwood 2lb residual valve to the front and Wilwood 10lb to the rear with a screw type proportioning valve to the rear (adjusted for full rear brakes). Every single part of the entire braking system is new and I double checked that everything is installed and adjusted correctly. The rear brakes just do not work well. I've tried two different residual valves to the rear and tried removing the proportioning valve.

If I test it in the grass, the fronts lock up and the rears will not unless I really stand on the brake pedal. And there is a delay from the time the fronts work to the time the rears do, which I think is built into the master for safety? On the road, the fronts lock up easily and the rears seem to do nothing. I had a situation last fall where a herd of deer ran out into the road in front of me. I hit the brakes and the fronts locked up and I skidded to the side of the road. Missed one by inches... Afterwards I looked at the marks and there were only two from the fronts.

I realize that locking the brakes is not desirable for most efficient braking, but for testing purposes I should be able to lock them in the grass easily I would think. Especially since the brakes were designed to stop a vehicle twice the weight.

Maybe the master cylinder is just not compatible with the rear? Maybe I need a master like the Corvette style? Any advice would be helpful.

 

[James C]

Never mind you said you checked that....

 

[railroad]

If the master cyl is high enough, I would take out the residual valves to the rear brakes. I will throw this out there, be sure the fronts are plumbed into larger reservoir side of the master cyl. Also, make sure the rear shoes are adjusted.
I ended up buying a brake pressure gauge to work out the brakes on another vehicle. I have forgotten the pressures you need to be seeing, seems like about 1200 on the low and 1500 plus for good braking. I got the gauge from Summit Racing. Good luck,

 

[2old2fast]

It may be as simple as not having your rear shoes adjusted correctly....for you guys who are setting up a brake system, while mixing & matching brake systems CAN be done successfully , why not use parts from the same vehicle ? TheO/E's have already done the engineering..i.e. pinto /mustang II granada ,monarch all use the same calipers/wheel cyl.'s , master cyl.'s...... S10 frt calipers /whl. cyl's/ master cyl.'s, an S-1o rear is popular for a T..... then you don't have to guess or listen to people who may or may not "know" what works together... K.I.S.S..
dave

 

[bobs66440]

I have the fronts plumbed to the large reservoir and the shoes are adjusted correctly. I agree regarding using all the same mfg equipment but this setup was recommended to me and is supposed to be common for T Buckets and is supposed to work together.... but not for me. The calipers are for mid size GM, like 70's-80's Camaro. Master is mid-70's 15/16" bore Mustang/Granada disc/drum and rear is mid-80's S-10 7/8" bore wheel cylinders. I would love to use a matched master cylinder but there is limited room under the floor and I'm not sure it would fit. I can check...but the bore size on that one is huge at almost 1.5"...

 

[Hotrod46]

With a 15/16 bore MC and 7/8 bore WC's. you're actually lowering the force applied to the rear shoes. My understanding is that GM purposely reduced the braking power on the rear of the S10 to eliminate rear wheel locking problems. They did this with small WC's and semi-metallic shoes. The harder shoes take more force to operate than the organic shoes and require more heat to work well.

I had a similar problem with the S10 brakes on my T. I was using Lincoln drums on the front, though. I have the bolt in WC's that were used on the later S10's. I was able to find some WC's off of an 82 Caprice that were 15/16 bore that could be modified to fit (lots of mods though). The larger WC's helped some, but didn't cure the problem. I switched to organic linings (the cheapest ones from Orielly's I think) and they helped some more, but the rear brakes still didn't do a lot of stopping.

Some T builders recommend plumbing a proportioning valve into the front brake lines and using it as a pressure regulator (which it really isn't). It works by lowering the pressure to the front brakes. I can't really say that it's wrong, just not the technically correct application of a prop valve. My solution would be to increase the braking force on the rear, not reduce it on the front.

I solved my problem by going to a dual Wilwood MC setup with a balance bar to regulate force applied to each MC. I think I'm running a 7/8 bore MC to the rear with 15/16 WC's.

In your case, the delay problem can be solved with a "hold off valve". This plumbs into the front brake line and prevents the front brakes from applying until something like 100 psi is reached. This lets the rear drum catch up to the front discs since they have farther to travel.

Not much can be done about the power difference without bigger WC's, more line pressure to the rear or just bigger/better brakes in the rear. You might want to think about a disc conversion. I can tell you that after I got my rear brakes working, the car stops better than you would think. So much of the braking is done on the rear of a T. They don't have the weight distribution that the cars we get our parts from do.

Mike

 

[choppedtop]

If the lines are full, why would it be a longer distance to travel?

 

[ellis8500]

The rear wheel cylinder pistons travel farther than the disk pistons. This is just a nature of the design.

 

[choppedtop]

Ok,,talking about the cylinder volume being greater. not the length of line travel. Gotcha. I am running 1" MC for my disc/drum setup only because that is what was recommended. Now on my roadster, I have the proportioning valve on the front disc, and have good braking with even stopping on gravel or wet pavement.

 

[bobs66440]

I was thinking about installing the proportioning valve to the front, but as you mentioned Hotrod46, decreasing braking to the front to balance out already poor braking didnt seem like the thing to do, but chopped top says it works, so maybe that's an option...

I had no idea this was such a common problem. Well, its good to know I'm not alone I suppose. I guess i have some experimenting to do. Thanks guys!

 

[James C]

The spirit guys put their proportioning valve in the front. Check this video here they start the proportioning valve at around 5:30

 

[Hotrod46]

The S10 rear is popular for buckets because it fits well, but the drum brakes GM shipped it with are not the best for our use. The disc brake rears are probably much better, but harder to come by.

Before I added the dual MC's, I was looking into adapting some large rear drums off of a full size pickup rearend. The late Ted Brown recommended big rear brakes and after driving my car for a while, I know why.

We've all heard the "75% of the braking force is on the front wheels" rule of thumb. That is probably true for a nose heavy sedan with a high center of gravity, but my T carries more weight on the rear than the front. I suspect most T's are at least 50/50. With their fairly low center of gravity, you just don't get as much weight transfer to the front. We put brakes on the front that are intended to stop a 3500 lb sedan and then throw on skinny front tires, so it's no wonder that there are brake bias issues.

The old VW front discs (or something similar) that were common many years ago combined with the biggest drums you can fit on the rear would probably be a better setup. This doesn't help the OP, but might be something for someone building a new car to consider. Just my opinion, for what it's worth.

Mike

 

[bobs66440]

The spirit guys put their proportioning valve in the front. Check this video here they start the proportioning valve at around 5:30

Hmm, thanks for that. Very interesting indeed. I understand the theory. The only thing about my setup is I have manual brakes and I really have to stand on them pretty firm to get it to stop as it is. I can't imagine cutting pressure to the front, how much force it would take then. I noticed they have a power booster so then it would not be an issue.

While increasing the rear braking ability as Mike says would be better, moving the proportioning valve seems to be my only option unless I want to spend many hours and dollars experimenting with different brake configurations. I wish I had the time and cash to do that. Maybe I'll try moving the proportioning valve, then if the pedal effort is too much, I'll add a booster. The end result should be the same...

Thanks!

 

[James C]

I'm thinking once the breaking is more even it will still require pressure but it will be perceived as better since it will be more evenly applied with the rear brakes actually working. When we used to do 3 wheel brakes on our dirt track cars it didn't seem like the pressure changed or got better even though we removed 1/4 of the brakes it just caused it to pull to one side (as we wanted) since they all worked correctly. If you look here @ page 7 of spirits manual top left corner they also put it on the front of their manual brake setup http://www.spiritcars.com/images/pdf/AssemblyManual.pdf I agree give it a try that way before converting. I'm sure it will be a PIA to do after the fact and it won't be cheap either. All my manual braked cars were easy to brake before and they weighed alot more than a T-bucket. It all has to do with pedal ratio and took more pedal travel. That said when I order my kit I'm going to do power brakes because she who must be obeyed has requested them since it will make her feel safer.

 

[bobs66440]

I'm thinking once the breaking is more even it will still require pressure but it will be perceived as better since it will be more evenly applied with the rear brakes actually working. When we used to do 3 wheel brakes on our dirt track cars it didn't seem like the pressure changed or got better even though we removed 1/4 of the brakes it just caused it to pull to one side (as we wanted) since they all worked correctly. If you look here @ page 7 of spirits manual top left corner they also put it on the front of their manual brake setup http://www.spiritcars.com/images/pdf/AssemblyManual.pdf I agree give it a try that way before converting. I'm sure it will be a PIA to do after the fact and it won't be cheap either. All my manual braked cars were easy to brake before and they weighed alot more than a T-bucket. It all has to do with pedal ratio and took more pedal travel. That said when I order my kit I'm going to do power brakes because she who must be obeyed has requested them since it will make her feel safer.

Thanks for the info. Maybe it won't be so bad after all. Time will tell...I'll let you know how I make out...

 

[Hotrod46]

Considering that the drums are self-energizing due to their construction, the pedal effort might not be increased at all if you can get them working. The front prop valve will probably help in your case since it will shift some of the stopping effort to the rear. I'm not a fan of it, but I can see where it could work and if it helps, that's great. Keep us posted, I'm curious.

 

[bobs66440]

Update...I moved the proportioning valve to the front and there is a HUGE improvement! Wow, I never would have thought, but that's the ticket. There is no noticeable difference in pedal effort and I was able to dial in the perfect amount of front/rear bias. I am very relieved as this will save me a lot of trial and error aggravation. Thanks everyone for your help!

 

[James C]

Glad it worked for you.

 

[Hotrod46]

Good info! Thanks for reporting back. Good news.

Mike

 

[Hackerbilt]

I have to admit I didn't expect it to work properly for some reason....but I'm having the exact same problem so I'm thinking I will give it a shot myself. I even have a new valve "in stock" to use.
Not a good feeling to hit the brakes and watch the fronts stop turning while the car continues on down the dirt lane hardly slowing down!
It stops fine on dry pavement but it's obvious the balance is way off.