When engine is running I can pump up pedal after two to three pumps but then hear a loud hissing

Okay, the noise could even be where the brake pedal and push rod come together. That makes sense.

We have two things we can try. Since you have already manually bled the brakes (and it sounds like everything was done correctly), you can try pressure bleeding them or try to bleed the master cylinder. Since you are seeing bubbles in there, (if there is air in the system) that sounds like where it will be.

Here are directions for that. If this fails and there is no air in the system, chances are the master cylinder is bad.

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1995 Chevy Truck C 1500 Truck 2WD V6-262 4.3L VIN Z TBI
Bleeding Master Cylinder
Vehicle Brakes and Traction Control Hydraulic System Brake Bleeding Service and Repair Procedures Master Cylinder Bleeding Without ABS System Bleeding Master Cylinder
BLEEDING MASTER CYLINDER
This procedure can be performed with master cylinder on or off vehicle.

1. Disconnect brake lines at master cylinder, if necessary.
2. Connect suitable lengths of brake lines to master cylinder and immerse other ends of lines in master cylinder reservoirs.
3. Apply master cylinder pushrod or brake pedal with full strokes until air bubbles have disappeared in reservoirs. It may require 20-30 applications to fully eliminate air bubbles.
4. Remove bleeding lines from master cylinder, then install master cylinder on vehicle, if necessary, and connect brake lines.

It is not necessary to bleed entire hydraulic system after replacing master cylinder, providing master cylinder has been bled and filled during installation.

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Let me know if this helps.

Take care,
Joe

I imagine someone does but I have not found anyone who does it.

Roy

You are welcome.

Always glad to help.

Roy

There are a coupe of things to consider. First, many four wheel ABS systems need to have a pair of valves opened in the hydraulic controller so the air can be expelled. You need a scanner for that. Air in that unit should not be a problem if the reservoir never ran empty. The master cylinder also can be replaced without needing to bleed at the wheels. That will also prevent air from getting down to the hydraulic controller. If the truck has the more common rear-wheel ABS, no special bleeding procedures are required for those systems.

The second thing to look at is most trucks and minivans have a wide range of loading variables, from empty to fully-loaded. To put the most braking power where it is needed, they usually have a height-sensing proportioning valve in the rear, connected with a link between the frame and the axle housing. If you tried bleeding the rear wheel cylinders while the truck was jacked up with the rear axle hanging down, that would put the valve in the position of having no load in the rear. The height-sensing proportioning valve would limit the amount of brake fluid pressure going to the rear brakes, and that could reduce the effectiveness of your bleeding attempts. Fluid flow might be slow enough that some air got stalled at a high spot in the lines. If the truck is on a hoist, put a pair of jack stands under the rear axle, then let the truck down to raise the axle. That will help brake fluid to flow faster to the rear brakes.

One other thing that used to throw us years ago is it is normal on most GM vehicles to be able to easily push the brake pedal almost to the floor when standing still in the service bay, but if you were on the road, that would equate with the brakes locked up and tires skidding. We were fooled many times into thinking there was still something wrong, then out of desperation, a test-drive proved the brakes were okay.

You have the RWAL system which was more common for the time. The dump valve and block valve are below and just to the right of the master cylinder in your photo. Next is the combination valve which is a standard part of all brake systems That is the brass block the two brake lines from the master cylinder go into. To the right of that is the ABS computer. That is the black rectangular box, about one inch wide. With this system, you bleed the brakes like normal, with no special procedures or scanner needed.

The next thing to look at is if the rear shoes are fully-adjusted. GM did not have much trouble with their self-adjusters, but look at the six "lands" on the backing plates. Those are the raised spots the shoes ride on. Most manufacturers bend tabs over on the shoe frames, then those tabs slide on the lands. Those should be lubricated with special high-temperature brake grease to prevent squeaks and wear. On GM's shoes, there are no tabs. The edges of the frames grind into those lands and cause grooves to form. Those can prevent a shoe from applying under light pedal pressure, and they can cause a shoe to stick and fail to release. Either condition can be overlooked and make it appear the shoes are adjusted properly, but when driving, they have to move too far to contact the drum. That will cause a low pedal.

While there are all kinds of things like this to look for, my suspicion is there is still air in the system. Did you bench-bleed the new master cylinder before it was installed? Have a helper push the brake pedal up and down a couple of inches repeatedly while you watch in the reservoir with the cover removed. If you see a few tiny air bubbles pop into the reservoir when the pedal is released, there is air in those lines. If that air did not get pushed down too far while bleeding previously, it will work its way out over time.

Also, look at the position of the shoes. Both of them must be contacting the large anchor pin at the top of the backing plate. If one is being held away from that pin, suspect the parking brake cable is rusted in the partially-applied position. That will prevent the shoes from self-adjusting. Next, look at the parking brake strut bar between the middles of the two shoes. You should be able to push that against the anti-rattle spring, toward the front of the truck about an eighth inch with thumb pressure. If it has no free play, again suspect a sticking parking brake cable.

I do not like suggesting this, but consider pedal-bleeding with a helper, or better yet, use a bleeder ball if you can find one. I have only used gravity-bleeding for many years because I always have to do everything by myself. With pedal-bleeding, you will get more volume at a faster rate. That will push air out that otherwise might get trapped in a high spot. Check too if clear new fluid is coming out at each bleeder screw. If it is still dark, air near the master cylinder has not made it all the way to the end yet.

It is important to never push the brake pedal over half way to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons in the master cylinder do not normally travel. Pushing the pedal to the floor runs the rubber lip seals over that crud and can rip them. That can result in a slowly-sinking brake pedal, and that often does not show up until two or three days later. I take every opportunity to mention this, but in your case, this does not apply because you have a new master cylinder. Keep that wondrous information for future brake work.

You also might consider using a hand-powered or air-powered vacuum bleeding system. Under a slight vacuum, any air will expand and be easier to pull out. Be aware though I have always been fooled the few times I have used these. If you have air in the system, the bubbles will come out and be seen as large bubbles or gaps in the fluid flow. If you see a constant, steady stream of very tiny bubbles, ignore them. That is air sneaking in past the threads of the bleeder screw.

Welcome to 2CarPros.

If the pedal is going to the floor and there are no leaks, the master cylinder is most likely bad. There are plungers in them that fail over time and allow brake fluid to bypass them. The result is the pedal will either fade, meaning if you keep pressure on it, or it will go directly to the floor. Since the fluid level isn't changing and is remaining full, this is my first suspect.

Take a look through this link. It explains in detail what happens. As far as the noise you are hearing, chances are the booster is making noise because the pedal is going so far down.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

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If you determine the master cylinder is bad, here is a link that shows in general how to replace one.

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Here are the directions specific to your vehicle. The attached pictures correlate with the directions.

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Master Cylinder
pic 1

CAUTION: Brake fluid will damage electrical connections and painted surfaces. Use shop cloths, drip pans and fender covers to prevent brake fluid from contacting these areas.

1. On vehicles with manual brakes, disconnect master cylinder pushrod from brake pedal.
2. On all models, disconnect all wire connectors from master cylinder components.
3. Disconnect brake lines from master cylinder. Cover the ends of the lines to prevent dirt from entering system.
4. Remove master cylinder mounting nuts, then the master cylinder.

CAUTION: Do not allow brake fluid to fall on the Anti-Lock control module, connectors or wiring.

For additional information see Notes, Warnings, and Hints. See: Brake Master Cylinder > Fundamentals and Basics

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Install

1. Bench bleed master cylinder.
2. Install master cylinder and mounting bolts.
3. Connect brake lines to master cylinder.
4. Connect all wire connectors to master cylinder (if equipped).
5. On models with manual brakes, connect master cylinder push rod to brake pedal.
6. Bleed brakes.

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Let me know if this helps or if you have other questions. Also, the link I supplied above (replacing a MC) explains how to bench bleed the new MC and system.

Take care,
Joe