I've been asked to post the template I used to make the C490 booster bracket. Here it is... on the right is the template I got from Ford Guy, which may work well on a '57 bird. On the left is my revised template that lines up correctly with the stock holes on a '55. The graph paper under each template is 1/4" quadrille, so you should be able to determine all of the relevant dimensions.
On my first extended drive with the new booster, I unfortunately had to make a panic stop on the freeway... offramp traffic had backed up onto the freeway, and I had little time to react. The front wheels locked up, but I didn't get the feeling that the back brakes were doing anything! I started to wonder if my brakes were plumbed correctly.
Gil Bumgarner highly recommends the use of a combo valve when installing disk brake conversions on early birds. He specifically references mpbrakes.com. I downloaded the T-bird disk brake conversion installation instructions from mpbrakes, and was interested to see that they use the same exact front brakes/spindles that I am using, AND that the instructions even include a picture of a C490 booster on the page that discusses power assist on the front only! They must know what they are talking about! Here is the diagram they include for how to plumb the booster into the front brake circuit:
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| Front boost only plumbing diagram from mpbrakes: the combo valve sees fully boosted front brake pressure. |
This is how I have been running the brakes with the smaller C3400 booster for the last 3 1/2 years. My only complaint is that it didn't provide enough boost to the front, and pedal pressure was unduly high. At one point with the old booster I decided to connect the "brake fail" switch on the combo valve to the dash oil pressure light. I noticed that when I applied the brakes, the light came on... and if I applied them hard, the light would stay on for a while. After doing some research on how combo valves work, I decided that this is normal behavior for an unbalanced system like this, and I simply disconnected the switch.
Note that in this configuration, the pressure differential portion of the combo valve senses the differential pressure between the front (boosted) and back (manual) brakes, and the valve spool moves toward the rear brake side of the valve. This turns on the brake fail light, and starts to limit pressure to the back brakes... which would be good if the back brakes had indeed failed, but they haven't. The differential pressure valve spool restores itself shortly after the brake pressure is released.
After my freeway panic stop, I started to wonder how changing the booster, with it's higher pressure to the front brakes, had effected this setup. Clearly there was now even more of an imbalance between the front and rear brake pressure in the valve, and the rear brakes were being even further restricted during panic stops... perhaps that's why I felt like I didn't have any rear brakes.
Since I want to use a combo valve to enhance safety and performance, AND I want to add boost to the front only, the question is I should have plumbed the booster A) in between the master cylinder and the combo valve (as shown in the mpbrakes instructions), or B) between the combo valve and the front brakes... as shown in the modified diagram below:
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I started to wonder if anyone else has thought this through. As it turns out, CASCO sells a combo valve for use with their disk brake conversion kit. Here is an excerpt from their 2015 catalog:
The master cylinder CASCO use is from Ford (with the rear portion in the front) instead of the corvette unit I used, and they use 4-piston fixed calipers instead of the single piston floating caliper units I (and mpbrakes) used. The plumbing attached to the combo valve in the picture is particularly illuminating... the bracket that CASCO includes hangs the valve out of sight underneath the battery heat shield, and the short lines attached to it connect directly to the master cylinder. If there is a front booster, it is definitely between the valve and the front brakes! They do require specifying power or manual when ordering. I'll send them a note and ask what the difference is. I sent a note to mpbrakes about their schematic, but they have not responded.
Here's a summary of the options:
Booster before combo valve:
- unbalanced pressures will trip pressure differential valve, potentially isolating the rear brakes in a panic stop
- Note that moving the differential pressure spool will adversely effect the proportioning valve, see cdx online eTextbook. The proportioning valve in meant to be disabled all together if the differential spool moves to the front fail position. I must assume that the proportioning valve will activate too early (limiting the rear brakes) if the spool moves to the rear fail position, which is what happens during a panic stop if the booster is before the combo valve.
- the front metering valve responds to full brake pressure, front metering delay is minimized. The implication of this is unclear. The combo valve was not meant for this kind of setup.
- this configuration was recommended by mpbrakes, but they won’t respond to my queries and I don’t really trust them anyhow. Front-only power brakes are very much a specialty setup, and I’m not sure they followed through very well on this.
Booster after combo valve:
- balanced pressures shouldn’t trip the differential valve, brake warning light should work correctly
- the front metering valve sees un-boosted pressure… there might be slightly more delay in activating the front brakes, but that isn’t necessarily a bad thing for stability… this would ensure that the rears activate first, an the metering valve intends.
- CASCO 2015 catalog, page 11 shows GM style combo valve with short plumbing attached to inputs (top); clearly the valve goes between the MC and the booster on the front circuit. I trust this more than the MP brakes install manual.
I replumbed the booster so that it is between the combo valve and the front brakes. I may not have fully bled the air out of the lines yet, because the pedal can go to the floor if the engine isn't running. They work (although with a low pedal) if the engine is running. I intend to get some speed bleeders and run a quart of DOT4 through the front circuit before doing anything rash.
Here's the response I got from CASCO, which is much as I expected...
ReplyDelete"That’s a good question. I have attached our directions for power combination valve for your review. The only reason we ask is because we supply different pre-bent tubes with the kit. So as you can see our arrangement places the combination valve between the master cylinder and the booster. But we have not considered putting it after the booster – our supplier simply offered it to us with this configuration.
My first thought is the valve is only a valve. As long as it can handle the boosted pressure from the booster it should not be a problem to have it after the booster. However in the case of a true combination valve - being able to isolate leaking problems to either the front or rear brake systems - it may warrant some consideration. I suspect the added “protection” provided by a combination valve may only be on problems after the combination valve and if so, one may want to minimize the leak risks before the valve. So in this case you could make the argument to put the valve before the booster. Of course I may be wrong on that point. Perhaps the safety piston will be triggered even if the leaking is occurring before the valve. I am just not sure.
Let me know what you end up doing and how it works out for you. Thanks for the email."
Here's what I wrote back to CASCO:
ReplyDelete"That’s pretty much what I expected. The only reason I can think that MPBrakes says to put the booster between the MC and the combo valve is that way the residual valve will see the boosted pressure, and the front brakes will activate sooner (relative to the rears) than if the booster was downstream of the valve. There may possibly be some delay in the booster “picking up” if it has to wait for the front brake input to overcome the residual valve pressure. I’ve noticed that the hydrovac has a little lag in applying the boost (compared to conventional power brakes), but it isn’t bad.
I’ve asked this same question of MPBrakes several weeks ago, but have yet to get a response. Right now, my t-bird’s brakes are configured the way you recommend, using an oversize Midland C490 booster, and I’m quite happy with the arrangement. As I press the pedal the rear brakes apply first, as they should, then with increasing pressure I can gradually feel the front disks come into effect, and then the added power assist. Modulation is actually pretty good, and the pedal feedback lets me know what’s happening. The pedal is a still a little heavy compared to a modern car, but it's noticeably lighter than when I was using the smaller stock booster (C3400).
I briefly had the plumbing the other way ‘round (booster between the MC and the valve) when I first installed the C490. I had one particularly hairy panic stop on the freeway that locked up front brakes, but it felt like the rears weren’t working at all! Luckily I didn’t hit anything. I suspect the combo valve was sensing the large pressure difference between front and rear circuits, and attempted to isolate the “leak” in the rear brakes! That rather scary experience was what started me asking questions and really looking into the “best” way to configure these brakes."