crholds442

I measured the front disc outlet pressure on a 1 inch diameter piston and 7 inch dual diaphragm booster and I only got 800 psi on my SSBC brake pressure guage. I have an automatic vacuum pump installed that puts out 21 Hg vacuum. This output is at least 200 psi under the required 1000 psi min output required for disc brake master cylinders. I sometimes wonder if I can stop in time in traffic. I don't have clearance for a 11 inch booster, but I think I can clear a 9 inch booster. Any thoughts on how to come to a quicker stop? Why aren't I getting 1000 + PSI at the master cylinder when I stomp the brake pedal, when I have 21Hg vacuum? SSBC tech. support says the min is 1000 psi for front discs.

oldcutlass

Is you MC a disc/drum or a disc/disc? You may need a 1 1/8 MC.

joe_padavano

That would make it worse. Increasing M/C piston diameter DECREASES line pressure for a given pedal pressure. Power brakes use larger M/C pistons because the power booster makes up for the higher required pedal pressure.

Of course, the real issue here is, why is the line pressure so low? Part of the problem is that POS 7" booster. The factory booster was 11". Even a dual diaphragm 7" booster only has about 3/4 of the area of the single diaphragm 11", so it will have less boost and thus lower line pressure for a given pedal pressure. In reality, dual diaphragm boosters loose some diaphragm area due to the way they are built, so usable area is even less. Why do people buy these things? Do the math. If you are trying to clear tall valve covers, then just get a hydroboost and eliminate clearance and pressure problems once and for all. If you have stock valve covers, throw that 7" piece of crap away and get a stock 11" booster.

crholds442

Thanks Joe. I suspected the 7 dual diaphragm booster. I have a 455 with Edelbrock heads, Erson high lift roller cam, Scorpion rockers and the tall cast aluminum "Oldsmobile" rocker covers. Is there any way I can put on a 11" booster? I'll happily ditch the rockers and the covers if I can stop quicker. You mentioned a hydrobooster…..I'll take a look at that.

Update: Summit has all kinds of Cardone rebuilt hydroboosters. Which one would be the best to adapt to the plumbing and pedal position on a 69 442? Does anybody have experience with this hydroboost adaptation? I'm tempted to buy standard height covers for 60 bucks and give that a try. I prefer to use the 11 inch booster as it was factory installed.

By the way, SSBC Tech. support avoided the issue of 7 inch dual diaphrams as providing weak/marginal hydraulic pressure on 4 wheel disc adaptations like mine.

crholds442

I'll try a Cardone rebuilt 79 Cutlass diesel hydrobooster to my 69 442. I tried regular height valve cover and it did not have any clearance for the Scorpion lifters. 11 inch booster is impossible.
I'll report on my progress. Lots of guys have engines too big for 11" boosters




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crholds442

I tried a hydrobooster from a 79 cutlass diesel, but the rod to the brake pedal was not convertible. Besides that, I would have to make up hoses specific to my 69 442. I wound up going to Performance Online (POL) with their A body conversion kit for about $600.00. I installed it to proper fit, drove it after getting air out of the system. I found out that when I go into a curve and apply braking, the steering jerks noticeably every time going into a curve and hard braking. This is unnerving and possibly dangerous on wet asphalt. The high pressure side hoses blew off the made up connections. When that happens all power steering and braking is lost. Yes, I did figure out how to properly make up the high pressure hose connections. This happened twice when braking hard. A real mess and dangerous in traffic. I had my steering box converted to a single, fast ratio which made it even more difficult to drive slowly with no brakes.

I've ordered a set of Wilwood high friction brake pads and an 8 inch dual diaphragm vacuum booster. Pretty sure it will fit, but I'll have to remove it to get my tall 4" valve cover off to adjust rockers or gasket. A single 11" diaphragm calculates 95 sq. inches, a 8" dual calculates 100 sq. inches and a 7 inch dual calculates 75 sq. inches. Nobody talks about the performance differences....not SSBC, not Summit.. Only Joe Pavdavano….when provoked. My warmed up 455 dream engine has cost me dearly in my time turning wrenches, and $$ in so many ways.. I hope the braking will match the engine performance. That's my story..so far.

crholds442

crholds442

8 inch Tuff Stuff dual brake booster works great, even without switching brake pads to high friction Willwoods. Note clearance to driver's side valve cover. This is the only decent option found after 3 years "futzing" around. Do the square inches math on diaphragm diameters. I hope this post helps someone. Note, POL has not yet sent me clearance to return 600$ hydrobooster conversion kit. I wish I had contacted Summit.....they may have had access to an A body hydrobooster kit. They always have accepted my returns when I didn't like the results or the fit. I'll check back when I swap out disc brake pads.

crholds442

The 8 inch dual diaphragm is working good with Wilwood high friction pads in the cast iron single piston calipers up front. I can further upgrade by installing aluminum SSBC dual piston calipers.

crholds442

This was a frame off resto that started Nov., 2010. Hydrobooster set up follows.