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By skygazer - 11 Years Ago
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I just learned something that I'd like to share. I have Grenada spindles/discs on my '55 'bird, stock rear drums with corv***e dual-circuit MC & combo valve. Stock Midland C3400 hydro-vac power booster on the front circuit only.
The car stops great, but the pedal pressure is a bit high. I know the booster is working, because I can feel the pedal go down when I start the engine with my foot on the brake. I'm running a Mummert 265 cam, but I haven't checked the vacuum at idle.
Anyway, I saw Gil Baumgartner's '57 'bird last weekend at the Santa Cruz pier. He was discussing disc brake conversions, and mentioned that he installed an upgraded booster. He said that the stock C3400 only develops about 500 psi, and disk brakes really need more like 1000 psi. He had installed a very similar unit, only with a larger diaphragm (8" vice 7"), which could develop these higher pressures.
This got me thinking and I did some research on the 'net. FYI, it looks like the unit Gil installed is a Midland C490BX. I can't find these readily available, but I can find the C490GX. It has the same dimensions, but a slightly different vent arrangement. The Cardone number for this unit is 51-9241 (edit typo in original post), and it seems to be available from AutoZone, Advance Auto, Summit, and NAPA. They are actually much cheaper than a C3400 from the t-bird supply houses!
I'll check my idle vacuum this weekend. If it's over 18", I will probably order one of these units. Much cheaper than an elecro-boost MC, and looks almost stock.
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By skygazer - 11 Years Ago
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I checked my vacuum at slow idle last night... only pulling about 14". I may be able to modify the tune a little to bring the idle vacuum up, but she's running strong in the current tune.
Now I'm wondering if a larger hydro-vac would even help.
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By skygazer - 11 Years Ago
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Did a quick search for "vacuum" on this forum. Looks like I should be pulling 17" with my current engine config. The needle flutters by an inch or two at idle, so I'll be checking the valve lash & doing a compression check. Hoping it's not something major. Just put 1K mi on the car during a road trip last week... hope I didn't burn a valve or something!
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By The Horvaths - 11 Years Ago
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You might check for a minor leak. Idle mixture screw(s) too, will affect vacuum. In fact, you want to tune them for max vacuum.
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By skygazer - 11 Years Ago
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Confirmed no leaks at manifold or carb. Could be internal carb issue, but I'm thinking it's more likely that 14" is normal for the Y265-S cam. Found a thread on hotrodder's forum about a SBC with a similar cam (224° @ .050, 112° LSA)... 14" was thought to be "normal". The Y265 has 110° LSA, so should actually have less vacuum... plus my 292 is significantly smaller than a SBC.
For the time being, I'm just going to enjoy the car. The brakes are a little stiff, but not unbearable.
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