I have air bubbles coming into the master cylinder fron my 1998 silerado truck what can cause that?

When bleeding brakes, always start at the one furthest from the master cylinder. On most vehicles this is the passenger rear. Bleed until there are no bubbles coming out then switch to the next furthest, drivers rear. Then the passenger front and end at the drivers front. Make sure that the fluid level doesn't go empty in the master cylinder or this will cause new bubbles to form in the system. If you continue to get bubbles, there must be somewhere that is allowing air to get into the system. If you have no leaks in any of the lines, I would have to think the master cylinder is sucking air.

try using a baby nose cleaner on master cylinder first. push cylinder down put cleaner in bottom master suck fuild , air bubble can be cause.

when you are done with a brake job it is always important to make sure you can slide the caliper both inward and outward by hand to ensure that there is no sticking, (that is with the pads in place and caliper piston completely depressed into the caliper) it is not rare to find that a piston that is very hard to depress back in the caliper is due to deterioration of the inner lining of the brake hose, if you have problems pushing the piston back into place most likely this is your problem. I have strong hands and is most vehicles I can push the piston back into the caliper by hand.

The brake master cylinder is directly in front of the steering wheel. If there is a hydraulic clutch the clutch cylinder should be bear it.

Sounds like you may have an air bubble in the master cylinder. The most effective way to deal with that is getting a master cylinder bleeder kit from your local parts store - make sure it has the metal clips that will hold the hoses in the master cylinder. Install line adapters after removing brake lines from the master. top up the fluid in the master and install hose clips, making sure the end of the hoses are in the brake fluid at least 1 inch. Slowly pump the brake pedal, making sure you bottom out the pedal and release SLOWLY so you don\'t suck air into the hoses - have a helper watch the fluid level in the master cylinder and the bubble production. The fluid level should not sink during this procedure - if it does, then you may have a faulty master cylinder. Pump until you have no more bubbles. If the bubbles don\'t stop, then you have a faulty master cylinder.

Hope this helps!!

Remove the lines running to it, and then unbolt it from whatever surface it's mounted to.
You will need to BENCH BLEED your NEW MASTER CYLINDER.
This can be done by taking some rubber hose and putting it from the holes for the lines, back into he reservoir on the new master cylinder. Make sure the new master cylinder is full of fluid. Put the master cylinder in a bench vice, and press in the plunger repeatedly with a screwdriver until no more air bubbles come out.

Then put the new master cylinder back to the mounting surface, and reattach the lines. Bleed the entire brake system.

When you changed the master cylinder did you bench bleed it if you didn;t there is air traper in it You need to take it back off you;ll need alittle kit there cheap screw lines inthe kit in to the side of the cylinder where the brake lines go put the hoses from the kit on the connectors then put the other end inside the master res Pht the cylinder in a vice fill if with fluid then take something that fits into the back of the cylinder and push and relese until you can see now more air bubbles coming from the hoses Reinstall and rebleed your brakes

Hydraulic clutch systems are renown for having airlocks in them once the fluid has drained out.
1.With the Master cylinder full have an assistant pump the clutch pedal acouple of times and keep it on the floor.
2. Release the bleed nipple on the slave cylinder to allow the air out and then tighten.
3. Assistant then allows pedal to come up.They may need to pull back pedal if it does not return by itself.
4.Repeat from #1 until no nore bubbles are seen coming out of the slave cylinder bleed nipple.
If you have a length of suitable size plastic tube that fits neatly over the the slave cylinder bleed nipple , the bubbles will be seen more clearly.
It could thae 8 to 10 pump sequences to clear the air lock. If the clutch still feels spongy, repeat the sequence after a couple of days driving.

Open the bleeder screws on your brake, and stick a gas line hose on the bleeder screw. Put the other end in a clear bottle of brake fluid, and pump the brake till all the air bubbles quit coming out. Then tighten the bleeder. If no bubbles come, you may have a faulty master cylinder.

Have you checked if there if air in clutch system?
Try bleeding slave cylinder..
a) Have someone hold down clutch pedal, then slowly loosen bleed screw on slave till some fluid escapes, noting if any air bubbles come out.
Repeat a) Until fluid is clear. (Keep an eye on master cylinder level)