When replacing a master cylinder is it necessary to bench bleed it?
Also does the isolation dump valve need to be bled that is on the master cylinder as well as all 4 bleeders?

 

To the best of my knowledge yes the master has to be bench bled and pretty much anything in the hydraulic system that can hold air in places you don't want it need to be bled as well.
Any air trapped in the system will not compress the same way as the fluid and can result in poor performance / failure of the brakes.

 

So I replaced the master cylinder and bled the brakes and even changed one of the back brakes lines but the brake pedal still is really low and the brake fluid going to the back brakes is not coming out as good as the front. Any suggestions on what the problem could be??? I also changed the wheel cylinders in the back brakes.
The truck is a 4x4 1500 chevy 89. Is the pressure in the back brakes going to be as strong as the front?

 

Do you notice any loss of oil?
the oil level in your master cilinder should be about 4 millimeters under the topping (there is an indicator that says "MIN-" in the cilinder). The seal with which the cilinder is closed should be intact. The master cilinder of those chevy's consists of 2 parts I believe. There's one for the front and one for the back.

 

Do you mean brake fluid or oil? The brake fluid is just below the top of the resevoir. The resevoir lid is tight and intact. There is what looks like a bleeding nipple just under the master cylinder itself should that be bled too? It is on top of a thing where some brake lines go into?

 

You put in too much brake fluid. Your master cilinder should be filled about 2/3th.
These are the steps you should take , with ABS system :
1) remove the hydraulic lines from the master cilinder and plug the outlets to prevent dust or anything else to get in there (I learned this from a manual, so I always tell everyone to do this because basically well...it makes me look smart :-D ). Disconnect the lines at the isolation valve.
2) remove the bolts from the isolation valve and then remove the master cilinder from the brake booster or firewall.

Then take the new MC (or clean the old one up :-D )
3)put the master cilinder and isolation valve back on the booster or firewall. When you tighten the nuts, make sure you dont over tighten them. Use a torque wrench. (and check your manual to know how much weight you can put on them).
then connect the brake lines and tighten them again. (again, check the weight and use a torque wrench).
4) fill the master cilinder for 2/3th !!!!! There's an indicator on the inside of both reservoirs.
Then FIRST !!!! bleed the master cilinder and THEN bleed the rest of the system.

This is the proper procedure. Good luck with it all.
Make sure you have a friend with you, cause you'll go nuts getting in and out of your car.
If you have done all this correctly and you still have that problem...well... Then there might be dirt in your rear brake system or the pumps might be broken , or you got a leak somewhere between the MC and your brakes, or your car is bewitched by your mother in law. (Never EVER underestimate your mother in law...)

 

Bench bleeding is highly recommended but not an absolute must. It just makes it easier and faster to bleed the system after installation. I have done it both ways on several vehicles; with or without bench bleeding. Some master cylinders bleed easy enough when they are installed in the car, and don't require bench bleeding. Some are tough to bleed even WITH bench bleeding, and really like to trap air.
Many new and rebuilt masters come with a flimsy bleeder kit included. Surprisingly enough, they work

When you are done bench bleeding, you have to fanangle the darn master in place and install it, and when you connect the hard lines, you inevitably lose some fluid. Which all too easily gets replaced by air.. and you are back to square one. Main reason why I don't see bench bleeding as a gift from the heavens, cure-all solution.

One tool that makes life easier is a power bleeder. It feeds pressure into the fluid reservoir, either air or pressurized brake fluid. You apply a little bit of pressure and crack one bleeder valve at a time, and watch for bubbles while the system bleeds itself. Keep one eye on the reservoir to make sure it has enough fluid in it at all times.

They also sell vacuum bleeders, but to me that sounds dumb, since the brake system is not designed to operate under vacuum, only pressure. Vacuum could cause seals to let air in. The seals are formed in such a way that they seal better the more pressure you put behind them. And vice versa!

 

One trick I've used with a lot of success is to bolt the M/C to the vacuum booster or firewall, then jack up the rear axle till the fluid in the M/C resevoir was level. Then bleed it on the car.

 

My truck's master cylinder is flat not tilted so i don't think that lifting the back up will help me but thank you. I noticed that when I took the lid off my cylinder that the grommet in the fromt has a bit of fluid in it. Is this going to be a problem? Will cause me to get air in my lines? There is no fluid dripping on the side or anything just when I wiggle it I notice some fluid. I also bled the brakes more and got some fluid going to the back brakes now but the pedal is still kinda soft. Does adjusting the help with this at all? There does not seem to be any leaks anywhere could it be a really bad vapour lock and I just have to keep bleeding? Any suggestions would be greatly apprecited.

 

Sometimes it takes quite a bit to get out all the air and dirty fluid.

If you've gotten the pedal to rise a little, you should be almost done. Usually it's not a mystery when you get the last of the air out of the system. You can really feel a solid pedal.

If all else fails, take it to a shop with a power bleeder and let them do their magic.