Master Cylinder Pressure Question

[motocrosschump]

does both spickets have the same amount of pressure coming out when applying the brake?

[Oldtruckfanatic]

That actually depends on the master cylinder and the type of brake system you have.. Are you talking drum drum or drum disc ?

[motocrosschump]

well its off a newer model truck. My old truck is drums all the way around. I still have the same proportioning valve if that matters.

[Oldtruckfanatic]

The Drum brakes take more pressure to push the shoes against the drum than the disc calipers take to clamp the disc. The master cylinder works by volume really and the proportioning valve handles the pressure so on a disc drum master the front brake side will move more fluid than the rear but the pressure is determined at the proportioning valve and a drum drum master will move a similar amount of fluid on both front and rear again the proportioning valve handles where the pressure changes from there. the combination valves do three things usually they handle pressure proportion between front and rear, Most have a hold off that keeps the front brakes from applying before the rear get enough pressure to start working and they usually have a residual check in them to keep a certain amount of pressre on the calipers and wheel cylinders so you don't have to pump the brakes to get the pads and or shoes out to the brakes. if you are using a disc drum proportioning valve on a drum drum system or vise versa the front brakes won't operate correctly. Also if you're using a disc drum master on a drum drum system the front brake side will be trying to move too much fluid for the wheel cylinders. hope this helps

[motocrosschump]

Damn you are one smart cookie.

[Oldtruckfanatic]

Not exactly lol, I read up on all that stuff to get my ASE certification and also if you go to master power brakes or stainless steel brake company's websites they sort of walk you through the troubleshooting and the function of the combination valves etc. I'm sure I didn't cover everything so if you're surfing around check out one of those places and you'll get tons of info. Thanks for the compliment though.

[hgs_notes]

The pressure in this hydraulic system at the master cylinder would be the same because the piston size in the master cylinder is equal front or back. Like oldtruckfanatic said, the difference is in the volume they push.

HG

[Longhorn Man]

the pressure is supposed to be higher for discs... not for drums.

[Oldtruckfanatic]

Wow I reverse a couple of words in the first line of a huge post and I have Andy the board police there to point it out to the world. Since you never make mistakes I'll let you handle the tech stuff from now on buddy.

[Longhorn Man]

OK, my fault... I didn't use very good wording, and it was intended to be more of an 'I thought' the pressure is supposed to be higher for discs... not for drums... not the cockey sounding correction.
It has been said many times on here that drums require higher pressure due to the springs that pull the shoes back, and that is not acurate.
I do make mistakes, I make plenty of mistakes, and i INVITE people to tell me when i am wrong. I make mistakes on here almost on a nightly basis.
Like responding here to get the correct info out to the person asking about the master cylinder.
I didn't even get into the fact that a master cylinder works off of volume AND pressure, I didn't get into the size of the brake line, the caliper piston, the master cylinder piston, the brake pedal leverage and it's effect on the whole thing...
While I do respect your testing and certification, I just wanted the info put on here to be acurate.
We've all learned things the hard way, and we've all gotten bad advice...through old style hot rodder circles at the cruise in, to online forums like this one..and even from the 'pros' who put out magizines each month.

in responce to your sarcasm, I'll make it a point not to add any info or correct any misleading mistakes in your posts anymore.
In responce to my misworded post... I'm very sorry and I can see how it came across wrong.

[motocrosschump]

At Any rate..I already had it mounted, just needed brake lines from prop valve to m/c. It works beautiful. Thanks Everyone.

[VetteVet]

Here's a great article that pretty much explains all that was stated.
http://www.hotrodheaven.com/tech/bra...kes3_index.htm