I'm getting ready to install a hydroboost. I've read where some have elongated holes to use their original MC. After a little investigation, I found that the Vaccum boost MCs are 1 1/8" bore while the Hydroboost ones are 1.3125". A 36% reduction in presure output. I also read where the hydroboost had burst some brake lines. This plus the fact that any modification to a safety item (which includes MCs) is ill advised at best. Lawyers love to point this out. I strongly suggest that if anyone installs a hydroboost, they spend the extra $50.00 dollars or so to use an F450 MC that installs without modification.

 

I think if those who experienced a burst brake line would chime in, I bet they would attribute the blow-out being more of an issue of 20 year old brake lines vs installing the hydroboost.

I've blown out more than 1 brake line in my lifetime with a vacuum booster, but I sure am not going to downgrade my brakes because of it under the claim that it had too much pressure.

Secondly, with the same brakes on the front and rear of the truck and the only difference being the MC (brake booster or hydro is irrelevant), stopping force is related to the PSI put on the wheel cylinders and calipers. To stop your truck from 60mph in the same distance with either MC will require the same PSI of pressure at the calipers/wheel cylinders to apply the same braking force to the brake pads. The larger OD MC will flow more volume per distance traveled and would therefore give you a higher brake pedal as there would be less travel distance required to move the needed fluid to get the brakes to squeeze. Pressure in the lines, hoses, calipers and wheel cylinders would be the same.

vacuum booster or hydroboost are just force multipliers so you don't have to provide all of the force by yourself. I believe you will find that the larger OD of the F450 MC comes from using 2.362" pistons in the calipers vs the 2.181" diameter of our F250/350 counterparts which necessitates more volume of fluid movement.

So, with MC output pressure remaining equal from both MC's for equal stopping distance, the force that changes is the INPUT force required to create the needed braking force. The large OD MC of the F450 will require more force applied due to the larger piston area to produces the same PSI output of the smaller diameter MC.

Larger brake caliper volume = greater brake fluid requirement = larger MC diameter = more input force to create needed pressure = factory hydroboost.

 

That's OK. So you're OK with modifying a MC to save ~$50.00. Any Roadside inspection could shut you down. I'm not going to risk it.

Just something about think about!

 

the f-superduty uses rear calipers too,not just wheel cylinders like we have for our drum brakes.

 

The cast iron master cylinders did not need any modification when I put mine on (2 trucks with the iron ones).

 

I would rather use a master cylinder designed for the calipers and wheel cylinders that my truck uses and sacrifice a little material (like 1/16" on each side maybe?) on the MC ears than use a completely different master cylinder that wasn't designed for the brake layout in place on the vehicle. By the time you assemble it you wouldn't even see that there was any modification to the ears.

Using a MC designed for rear disc brakes with rear drums may prove interesting!

 

Same here. The one that was in my 86 bolted right to the hydroboost setup I installed. So did the replacement MC I installed a week or so ago. No modification needed at all.

 

And replacing my cast iron MC with an aluminum one only required ~1/16" of material removal per side I would say (hey whatya know.. same as needed to put aluminum MC on hydroboost!). But if you find the thought of removing a little material completely unacceptable, try swapping in the proper cast iron MC instead of the superduty MC.

 

IIRC the big differance between a master cylinder built for Disc brakes and for drums brakes is the size of the reservoir. The disc brakes require more fluid to operate. There may or may not be a differance in residual valves. Used to be 10lb for drum and about 2lbs for disc. The proportioning valve should provide that function now. Usiing the disc/disc master should cause no problems, it is possible ir could give a mushy peddle, but not likely. However, you should never use a master cylinder designed for discs on drums, in theory anyway (I have done it on a VW powered trike disc conversion with no problems).