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Discussion Starter · #1 ·
I have seen a couple posts with regards to the angle of the master cylinder. Hmmm. Why are some parallel with the engine, some positioned as an angle up, and some aftermarket set-ups too high and hit the hood? Hopefully, this may help a couple of you and answer a few questions.

The use of the power brake booster seems to be the answer.

The standard, non-disc front brakes, uses a dual master cylinder with each port for each line having a check valve. When brakes are applied and then released, the springs behind the check valves will close the valves off, and 8-12 pounds of line pressure is maintained/kept in the brake lines. For you hot rod guys, this is called a "residual brake line check valve" which is often installed somewhere near the master cylinder on older cars/set-ups because the older master cylinders do no have the check valves. The 8-10 punds of pressure in the lines keeps the wheel cylinders "loaded" and respond faster when you apply the brakes. OK, that's for drum brakes.

How about disc brakes? Right from the '68 Pontiac shop manual. NOTE: WITH CALIPER DISC BRAKES, NO CHECK VALVE IS USED IN THE SYSTEM FOR CALIPERS. Stated another way, "NO CHECK VALVE IS USED IN THE MASTER CYLINDER SYSTEM AT THE BRAKE LINE GOING TO THE FRONT CALIPERS." WHY? The drum brake master cylinder applies balanced pressure to operate both the front & rear brakes, not so with front disc brakes. More on this later.

Ah HA! This may be one of the big contributors when some do the disc swap or upgrade on the front drums and then do not use the correct master cylinder, regardless whether they use a manual or power system. We all want to save a few bucks, right? Who knew?

BUT WAIT!, there is more to this. Again, '68 Pontiac Service manual points out this little tidbit when the car has been equipped with the Heavy Duty Power Brake Dual Chamber Booster (used on full-sized Pontiac's like Police/Taxi options). NOTE: BOTH HYDRAULIC PISTONS MUST RETURN FAR ENOUGH TO OPEN BOTH COMPENSATING PORTS, BUT THE HYDRAULIC PUSHROD SHOULD ALWAYS REMAIN IN CONTACT WITH THE REAR PISTON." Said another way,"WHEN YOU RELEASE THE BRAKES/PEDAL, THE MASTER CYLINDER'S INTERNAL PISTONS MUST RETRACT FAR ENOUGH BACK TO OPEN THE COMPENSATING VALVES OF EACH BRAKE LINE ALLOWING BRAKE FLUID RETURN, AND THE ROD PROTRUDING FROM THE BOOSTER MUST BE CORRECTLY ADJUSTED/MEASURED TO REMAIN IN CONTACT WITH THE CUP FOUND IN THE END OF THE MASTER CYLINDER WHICH IT PUSHES AGAINST WHEN BRAKING IS APPLIED. IF THE ROD IS EXTENDED TOO MUCH, THE COMPENSATING PORTS MAY NOT BE CORRECTLY/FULLY OPENED - CAUSING BRAKING ISSUES."

Again, this is noted for the HD Dual Diaphragm Bendix brake booster available for the big Pontiacs, as this may apply to some of those aftermarket dual booster set-ups - drum, drum/disc, disc/disc/. The above note is specific to the power brake boosters and drum brakes, BUT the rod stick-out of the power booster should be measured any time you piece together a master cylinder/power booster set-up, and could be something to check even if you purchase/install a set-up that was previously assembled - because you were told you should be able to bolt it right up and go. The amount of pushrod stick-out may be MC and/or booster specific, so find out before slapping it all together and then trying to figure out what is going on, what other part needs to be tested/replaced, or slamming the kit supplier for Chinese junk - when it may have been an installer problem because not enough questions/information was looked into before purchasing/installing a braking system.

Reading the Service Manual (how many of us do this, right?), the disc brake option uses a different master cylinder. As pointed out earlier, it does not have both residual valves at the end of each brake line port, AND the MC also a different bore size.

A Metering Valve is used to the front line going to the disc brakes (for both manual/power disc) and prevents application of the front disc brakes until about 75 PSI is built up in the system so that the rear brakes contact the drums at the same time - otherwise the front disc brakes will apply first or lock up causing erratic braking or the need to pull out the clean underwear from the glove box and swap them out on the side of the road. Since residual hydraulic pressure cannot be tolerated at the calipers, a residual check valve is only used for the rear brakes. The Metering Valve is located below the master cylinder and both the lines coming from the rear reservoir (drum brakes) and out of the Metering Valve (discs) connect to a Brake Junction Block (just the same as drum/drum brakes) having the brake light switch and piston/valve that closes off either the front/rear braking system in the even of a brake failure, ie busted/leaking line or wheel cylinder. The movement of the piston/valve within the brake light warning switch activates the dash brake warning light indicating a problem.

Drum Brake MC - Bore - 1.00" Both brake line ports have residual valves.
Disc Brake MC - Bore 1.125" Only rear brake port has residual valve.

Pic #1 - Power Brake Booster - uses a different mounting bracket than manual brakes.
Pic #2 - Exploded view of the manual MC and the residual valves. ID circled in Red.
Pic #3 - Disc Brake Set-up and all its parts.

Rectangle Font Parallel Slope Technical drawing
Font Line Parallel Rectangle Slope
Motor vehicle Organism Font Auto part Art
 

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Does the angle have anything to do with clearing the valve cover? My aftermarket kit from MBM came with a straight master bracket and it was to close to the cover even being a dual 8" so they sent me the other one pictured in the install manual, also there's a hole between the chambers in the master so I have to fill it up to the top in the rear chamber to have a enough in the front chamber but I know this is a cheaper aftermarket setup.
 

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Discussion Starter · #3 ·
Does the angle have anything to do with clearing the valve cover? My aftermarket kit from MBM came with a straight master bracket and it was to close to the cover even being a dual 8" so they sent me the other one pictured in the install manual, also there's a hole between the chambers in the master so I have to fill it up to the top in the rear chamber to have a enough in the front chamber but I know this is a cheaper aftermarket setup.
The difference in brake pedal pressure needed for manual brakes versus power brakes.

Here is what I found that explains this;

"
The primary reason is for correct brake pedal geometry. Back in the day, most GM vehicles were offered with both standard manual brakes as well as optionally with power-assisted brakes. The optimum manual-brake pedal-to-master cylinder pushrod ratio (aka "pedal advantage") is around 6:1, compared to about 4:1 for power-assisted brakes which don't need as much pedal advantage because they "boost" the force generated by the average human leg. GM used a common brake pedal with two pushrod holes located about 1 to 1 Inches apart; the upper hole was for the 6:1 manual brakes and the bottom hole yielded the power-brake 4:1 ratio. When the brake pushrod was installed in the lower hole for use with power brakes, achieving the proper pushrod arc of travel, center of force, and proper alignment with the brake booster/master cylinder assembly piston centerline required a firewall mounting bracket that positions the assembly at a fairly sharp up-angle

On a classic GM pedal with two pushrod holes, the top hole is for manual brakes; the bottom is for power (which needs less ratio). The booster/master cylinder mounts at a steep up-angle to properly align with the pushrod. Pedal advantage is determined by dividing A (the pedal pivot to the pedal-pad center push-point) by B (the distance from the pivot to the pushrod hole).

Over many years, this resulted in substantial savings for GM in parts and manufacturing costs, cutting the required number of brake pedals for any given model in half (from four to two: one for automatics, another for stick-shift transmissions). Drilling two pushrod holes in the pedal also allows a common firewall master cylinder/vacuum booster mounting pattern and a single large center-hole with any brake option. An additional benefit is increased fender and/or valve-cover clearance, critical with old-school big-block engines. (Even so, on many installations including Vettes and Camaros the rear of the big-block driver-side valve covers was cut off at an angle for power-brake booster clearance.)

Generally the "up-angle" on a power booster/master cylinder installation was at least 17-degrees, but if you look closely at an old stock manual brake set-up, it, too, often had a slight upward angle—about 6 degrees."

Wheel Font Elbow Slope Rectangle
 
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