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Brake problem (almost everything replaced still a soft pedal)

I have a 1988 Nissan Pickup with a 2.4 motor. Recently it failed inspection due to the brakes. So I have replaced almost everything on the brake system. New front lines, pads, rotors, rear shoes, rear cylinders, drums, springs, adjusters, and levers have all been changed. After changing all this I had a very soft pedal. I bled the brakes three or four times with no affect. So I took it to the local mechanic. He said the master was bad. I purchase a new master it is installed and still have a very soft pedal. I bled the brakes again and have some pedal back but not 100%. First push goes to the floor, second it gets better. I have turned the truck off, pumped the pedal till it gets hard and then turned the truck on. The pedal will then go to the floor. Take the vac line off the booster will give me a very hard pedal but still soft brakes. When I push on the pedal and the truck is running there will be a change in idle then go right back to normal. Every time the brake is pushed the idle will change.

Can there be a small leak in the booster? Can the check valve in between the booster and the motor be bad? I am at my wits end and my wallet is starting to hurt a little.

A leak in the booster would give you a hard pedal rather than a soft pedal. The booster only helps push.

Was the new master cylinder bench bled before installation?

You probably still ave air in the system. It takes a while to completely bleed the brakes with so many parts replaced. You also need to do them in the correct order (but I’ll bet you knew that).

And Mountainbike’s question about bench bleeding the master cylinder before installation is a good one.

Try having the system pressure bled.

An often overlooked part of the system are the short rubber brake hoses between the hard line and the caliper.
As these hoses age they get soft, and will bulge when pressure is applied causing the soft pedal. There are performance hoses available, but I doubt you need that. Just try some new factory hoses.


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The OP said the “front lines” were replaced, leading me to believe it was the flex hoses. However, there is still the rear line to consider.

I am not sure if the master was bench bled. I was getting so irritated I took it to a mechanic. However this mechanic told my wife the brakes were okay when they weren’t so I have refused to go back to him. I have pressure bled the system myself but still got the soft pedal. The truck at the moment is at another mechanic whom has done good work for us before. He was the one who “thinks” it is the booster. The rear lines are complete metal, no rubber at all. Front lines from the caliper to the hard lines have been changed in the front. What does the drop in idle when the brake is pushed mean? Someone I talked to mentioned maybe a bad check vavle. I am ordering the new booster and check valve as these are the only things that haven’t been changed. The brakes were fine before all this work was done. At the bottom of the pedal I can stop the truck as I had to drive it quite a ways to get it to the mechanic periodically pumping the brakes to get pressure back. I thought it could have been that the rear shoes weren’t adjusted properly. I adjusted them only to get my emergency brake back. Still a soft pedal. Thanks for your comments so far.

There is a flex line in the rear. Look again.

You may have a bad rebuilt master cylinder. It happens.

When the brakes were tested the first time it was shown that the rears were working 27% and fronts weren’t working at all on the first push of the brakes. I don’t know if this helps at all.

I would check the front calipers for binding and make sure the calipers can float properly. It sounds like the pads could be backing up all the way and have to be forced back out against the rotors.

Once you have “pumped up” the brakes, does the pedal get firm or is it still soft?

Are the rear brakes properly adjusted? Rears that are way out of adjustment can mimic a weak master cylinder of a system with air in the lines.

Rears that are way out of adjustment can mimic a weak master cylinder of a system with air in the lines.

Good point. So can shoes that don’t fit the circumference of the drum.

Here’s something interesting.

On models so equipped, begin at one of the bleeder valves on the master cylinder. On models where the master cylinder has no bleeder valves, begin at the load sensing valve attached to the right frame rail, in front of the fuel tank. After bleeding the sensing valve, bleed the rest of the brake system.


Rear brakes have been adjusted. Shoes are the proper ones, matched them up with the originals. Brakes get firm upon pumping them.

Consider two things: one being the fact that you’ve interupted a normally sealed system (Brake system), and two, you’ve had too many mechanics looking into the system.
If you’ve replaced all this hardware yourself you’re intelligent enough to limit the amount of people who look into your system - trust yourself. Think about it, you’ve replaced all this hardware yourself and you’ve fallen back on a source which you stole business from. Sure that shop will tell you there are more things wrong with it! If it was the booster, you would have had these symptoms before you replaced all those pieces of hardware and you would hear a humming sound when you press the pedal. I don’t think it is the booster. Make sure everything that attaches to the booster is tightened correctly and seals are good - if any (I don’t know your car). Because there is a change in the idle when you press the brake pedal, there absolutely is a vacuum leak somewhere. Very unlikely to be the booster itself.
This is one of a couple of problems. The other problem is air introduced to a normally sealed system. Us at-home mechanics don’t have those expensive tools contibuting to the overhead of the local shop. Rent a pneumatic air bleed system or have a trusted shop do specifically that ONLY - bleed the brakes with their pneumatic bleeder, then have them show that you have a small opening in the system somewhere (From the replaced lines and new master cylinder) and make them show you where that small opening is (That is something their pneumatic pressure bleeder will do for them). I bet you probably didn’t bench bleed the new master cylinder before you installed it, right? Don’t feel bad it is an honest mistake, and we learn from experience. You are probably introducing air back into the system perpetually from the poorly bled master cylinder. The pnuematic pressure bleeding will cure this, no questions asked.
After you are absolutelty certain that there is no air in the system, their may be a pedal travel adjustment which you can asjust yourself fairly easily. It is different per make of car where the adjustemnt is done. Most of the foreign cars will have the adjustment somewhere inside the car next to the pedal. Most domestic cars manufactured in the US will have the adjustment on the master cylinder rod.
After all this, if you haven’t replaced the proportioning valve you might as well replace it now just to be sure (Before the pneumatic bleeding). Also keep in mind that some pick ups have load sensors located somewhere below the bed. I don’t know when they started putting these laod sensors on so check this also.

You have probably done this, but I would go through the bleeding process one more time. Make sure you go to each component sequentially. Make sure you fully bleed each part. I assume you don’t have brake warning light on?

What order are you bleeding the brakes in? You should always start with the wheel fartherest away from the master cylinder.
RR, LR, RF, LF etc.

Well I have a pressure bleeder here at the house that I was using to bleed the system. I started RR, LR, RF, then LF. I was getting frustrated with the brakes and that is the only reason this truck was taken to the first mechanic. This mechanic then told the wife the brakes were fine when he knew they weren’t. This left my wife on the German Autobahn with a truck and no brakes. If I would go back to this mechanic there would be hell to pay. We then took the truck to mechanics who have serviced my wife’s cars in the past. I think one problem is that the mechanics assume my work is good and they haven’t checked anything that I have done. I believe my work is good enough but something is wrong. I have checked underneath the bed and have not found any load sensing sensors as this truck is pretty old.

I did not install the master cylinder and I am assuming this guy did nothing but stick it on and fill it up. I am ordering all new booster and check valves this way everything will be new and I can check these off the list. I highly doubt that the booster is bad but it being 20 years old probably wouldn’t hurt to replace it and the check valve.


Shoes are the proper ones, matched them up with the originals.

Given the problem you’re trying to track down, I would go one step further. If you take off all four of your rear shoes and, by hand, place them inside the drums, do you get any clearance or rocking at all? If you do, then you’ll need to get the shoes ground to fit the drums (does anyone do that anymore?), or buy new shoes that fit correctly.

It’s rare to see the shoes and drums mismatched, but you’re chasing a rare problem.

While on the subject of rear shoes and if it matters with the type of shoes you bought, do you have the front and rear shoes installed in their correct positions (and not swapped with each other?) If two of your shoes have more material on them than the other two (longer length of friction material bonded/riveted to the web), make sure the one with less material is toward the front of the car and the one with less to the rear. This may not be an issue since it’s common these days to see the same length of material on each front and rear shoe.

You have been given good advice thus far- one thougt related to rear breaks- Were fr. flex hoses leaking, or were they blocked (hapens on bends) if blocked, you may have been way out o wack before repairs as to breaking fr/back. Also- does she pull? A low teck way to find where air (or bad/ mis adj. caliper or cylinder) is to put it on jack stands, have someone put moderate push on pedea, and look for what wheel ya can turn easer by hand. Good luck- don’t overthink, ain’t rocket science, though very fustrating!