So here's the deal... working on my project (MGB, 302/T5 swap)... using a rubber bumper shell so have the later style pedal box that's set up wtih the brake booster. I don't like that look. I want to eliminate the booster so, i'm thinking the answer is...
1. Find someone to trade my pedal box & brake booster assembly with
2. Get a master cylinder that'll work with my brakes (4-wheel disc w/Wilwood front & C4 Vette rear) & fit behind the pedal box
If I can't find a CB pedal box I'm thinking I could modify the RB pedal one. Looks like I'd just have to drill the holes to mount the master cylinder since they're not there.
So, two questions...
a. anyone know if the pivot point on the brake pedal would need to also be moved if i go this route? If so, up or down & by about how much?
b. recommendations on a master cylinder that'll fit behind the CB pedal box & have sufficient volume to work with disc front & rear?
Rob, I don't know what your budget is but since you want to modify the master cylinders anyway to use with a 4 wheel disc system why not mount something like a Tilton assembly and get a selection of cylinder sizes to work with along with the balance bar for front to rear bias adjustment. You should be able to get it all for around $500. There are different mounts that would mount the master cylinders either in front or to the rear of the pedal pivot. http://www.pegasusautoracing.com/productdetails.asp?RecId=1097
3533.jpg
I really hope I can do this for way less than that. Plus, i'm not sure I want a bias bar -- this is a street car so I'm thinking a proportioning valve is plenty sufficient to set the bias & it shouldn't have to be adjusted much or often after that.
My new MGB master cylinder arrived today... but if I were starting from scratch here's what I think would be nifty:
I'd use dual master cylinders with a bias bar for brakes - but I'd make room for them by setting up a cable-operated clutch! Compared to a hydraulic throw-out bearing, a cable operated clutch would save a couple hundred bucks. It would certainly be simpler to maintain or repair.
The result would look sorta like this: (Will Holoman's old car... check out the brakes!)
(http://www.britishv8.org/MG/WillHoloman/WillHoloman-B.jpg)
The reason Will didn't have a clutch slave is that he fitted an automatic tranny. I wouldn't do that. The dual masters shown here were purchased as a single unit from "CNC"... dune buggy stuff, I think. Since the car was a '65, it originally came with a single-circuit brake system.
Rob, I'll have my MGB master cylinder out for replacement this weekend. I can measure pedal geometry, etc. if you need that info. E-mail me any specific measurements you need.
Incidentally, while I'm messing with my new master cylinder, I'll also re-route the brake lines. I got to looking around in the photo gallery for ideas... So, now I have a question about BARNEY. Steve Carrick is a magician! He somehow (and for some reason) made his brake lines dissappear. What's going on in this picture?
(http://www.britishv8.org/MG/SteveCarrick/SteveCarrick-Q.jpg)
QuoteIf I can't find a CB pedal box...
Don't forget to post a "wanted to buy" ad in the message board's classified ad section...
RE: What's going on in this picture? [Steve's brake line disappearing]
I'm pretty sure he's simply routed the brake line down through the existing hole -- probably to a proportioning valve -- in the cockpit. But, i'll be really interested to see what he says.
RE: I'd use dual master cylinders with a bias bar for brakes...
I've heard you say that twice now... Edumacate me... what's behind that? E.g., what does it buy you that you don't get with a proportioning valve. I understand the dual cylinder/bias bar enables different size bores for front/rear & probably better fine tuning of the bias, but if a single master [tandem circuit] master + proportioning valve lets you lock up the front & adjust the back so it comes close, but doesn't lock up, I'm more curious about the practical benefits of a dual master/bias bar set up for a street car? Seems like overkill & extra cost/complexity?? Am i mising something?
I haven't ever installed a proportioning valve, although I know that a lot of smart people swear by them. Probably they're just fine for most applications. They're certainly easier and cheaper to install...
My bias against proportioning valves dates back about twenty years to whenever I first read Carroll Smith's "Tune to Win". (All of the Carroll Smith books are outstanding, by the way.) Chapter 14 was titled "The Peculiar Case of the Large Sedan". Don't let the term "large sedan" put you off - Carroll Smith mostly wrote about open-wheel race cars, so this chapter is probably applicable for anyone building/racing a car over ~1200# or with more than ~4" of ride height. There's no chapter titled "for little British sports cars". Anyway, Smith wrote:
QuoteYou will also need a twin master cylinder and bias bar setup which is best purchased from Tilton. Don't fool around with proportioning valves - there are no suitable ones available - the ubiquitous Kelsey Hayes unit has too much hysteresis for racing and the rear brake line pressure doesn't release quickly enough.
Which I know will beg the question: "What is hysteresis?" You may already know that it's a word engineers use to describe a system where behavior or status doesn't just depend on physical inputs, but ALSO depends on historical state. I think specifically what Smith was implying is that having the proportioning valve's knob at a given position (say "1 o'clock") doesn't actually provide a predictable amount of forward/rearward brake bias. It depends either on what position the knob was at before it was turned to that position (say... "4 o'clock") or probably (IMHO, because it relates to the "doesn't release quickly enough" remark) the amount of brake bias may depend on whether you were last pushing the brake pedal half a second ago or four seconds ago. Possibly, pumping the pedal repeatedly might ratchet the bias rearward.
Does that explanation make any sense?
In my particular installation, fitting a proportioning valve doesn't seem to make much sense because I need relatively more REARWARD bias than I've got at the moment, and the proportioning valve can only "remove" rear braking effect (unless you fit it on the front circuit).
A bias bar system will feature simpler master cylinders (presumably more reliable) and would facilitate re-engineering of the system in the future (for example, if I upgrade calipers or fit rear discs.) Of course I know that my car isn't really a race car... but I like to emulate race cars where it's feasible.
Great info!
Now, back to the question of whether I'd need to move the pivot point if I use the RB pedal box with the CB dual circuit master cylinder... anyone know?
Rob,
When I was switching my braking system from power to manual, Bill Guzman gave me the following advice which is very relevant to your question.
A typical RB brake system was designed with about a 4:1 pedal ratio (typical with power/vacuum assist systems). If you are switching to manual brakes, you'll need about a 6:1 ratio. For a RB brake pedal, this means raising the pivot point up about 3/4".
Rob,
Just clarifying my previous post, the pivot point for the pushrod/clevis is essentially raised by 3/4". Of course it follows that the M/C is also raised the same amount (which might effect hood clearance issues).
That's the info I was after Graham! Thanks. I wonder if the actual pivot point on the CB pedal is closer to the bottom to make the same ratio w/o raising the level of the master cylinder? I bet I can scare up a CB pedal to compare them... should have thought of that to start with.
Ok, i took the cover off my RB pedal box... Rubber bumper brake pedal pivots at the very top & current connection to the booster is below the mid line of the clutch cylinder by about 3/4" so it looks like you can raise it by that much & you'll end up with the brake master mounted at about the same height as the clutch master. But, I don't see how I could mount it on the back side & have it push the rod in. With the pivot at the top of the pedal, no matter where you connect it, it's going to move toward the front of the car.
Then I rummaged through some parts & found a CB brake pedal (turns out I have two full sets of CB pedals & two pedal boxes -- although one is fairly rusty. One of these days I'm going to catelog all the stuff I got with the parts car I bought!) Anyway, chrome bumper pedal is different. Pivot point is about 1 1/2" or so below the top. So when you push on the bottom of the pedal, the top moves toward the cockpit -- & pushes the brake master rod in. So definitely looks easier to use the CB pedal box than try to convert the RB box to work. I'll need to adjust the shelf the box sits on because the base of the box lands over the hole, but that'll be way easier to address than modifying the pedals or pedal box.
So with that solved I just need to decide on which master cylinder to use.
"... or with more than ~4" of ride height. "
Is this the 4X4 section of the Forum?! :)
Rob, I just had a "V8" moment and had to smack my head. We have a CB pedal setup from the project Roadmaster up for grabs as we're upgrading to a RB assisted unit I sent Jim.
Quote"... or with more than ~4" of ride height. "
Is this the 4X4 section of the Forum?! :)
You got me there!
Maybe I should've written "... or with the driver's ass more than three inches above asphalt."?
Bill, do you recall the condition? If it's in good condition, i'll buy it from the Roadmaster project as the ones I have are pretty crusty. I could blast & repaint one of them (the other has too much cancer to use w/o fixing it) but I'd rather get one that's ready to go & deal with cleaning/selling excess parts when i'm on the road! Let me know & if it's in good shape I'll connect w/Jim on the Roadmaster thread.
Rob, I've never seen the car or the original parts other than in the photos Jim posted but the assembly was shown in the early set of photos when he first got the body and did a trial fit on the engine. You'll have to check with Jim as to their condition. You should also take a look at Jim's take on twin master cylinders for the B, he did a nice job of adapting a second cylinder to the original pedal assembly in his car, would definitely work for you and allow you to use the correct size cylinders for both front and rear if you want to go that way. He described and photographed the installation very well on his page in the newsletter. http://www.britishv8.org/MG/JimBlackwood.htm
Rob,
It was a fairly solid car that belonged to Steve DeGroat. He would know about the condition of the pedals. I've been to Jim's & help work on it, but I didn't really pay any attention to the pedal assembly.
Wilwood $315.00 at Speedway Motors as shown
3 difference size Master Cylinders to choose from
91029974_R.jpg
Adjuster for $47.99
8353404990_L.jpg
Good info Will. I ended up using a CB pedal box & pedals. Had to modify the shelf where it mounts a little, but not much. Seems to work fine.