Brake hydraulics vs. pads and shoes?

I have a Volvo V70, 2007. I took it in for routine replacement of front brakes (shoes and pads). Aside from the grinding sounds, brakes were functioning normally. When I got it back, I observed that the pedal was going to the floor. Upon pumping once, I’d get some pressure, pumping again, I would get mostly full pressure. But after 30 seconds or so of not being on the pedal, it would return to its original state.

I’m for sure a novice, but I do have a very basic understanding of the hydraulic system. My understanding is that the pads and shoes are not connected to the hydraulic system. Is this correct? in the process of changing parts, in there an opportunity for air to be introduced into the hydraulic line? The mechanic is telling me there is a problem with the master cylinder. If this is the case, why would this show its head after replacing pads and shoes?

I will also add that I the difference between hydraulic pressure before
and after the service is drastic. I have considered the possible that
pressure was diminishing gradually and I didn’t notice. I’m confident
that this is not the case. The difference with the brakes feel now is

It is pretty egregious to suspect or suggest that the mechanic intentionally put air in the line to sell me a new master cylinder, so I am not jumping to that conclusion, but I am always trying to be as aware and educated as possible.

Lastly, I’m not sure if this is important, but it was only the front brakes that I had done.

Any input is appreciated.

No, The pads and shoes are applied by the hydraulic cylinders - caliper pistons or slave cylinder - and press the brake friction material onto a drum or disk connected to the wheels.

You can’t know that without pressure gauges on the brake system itself. You are describing the force required for you to apply to the pedal to stop the car.

Since the fronts only were done, the disk “pads” were changed, not the drum brake “shoes”. During that change, the front caliper pistons had to be pushed back to install the new, thicker pads. Several things could have happened here.

The mechanic could have introduced air into the lines and he forgot to bleed the air out the lines. By pushing the fluid back could have cause a marginal master cylinder seal to fail and yes, you may need a new one, not his fault, its just time. He also could have caused a seal or valve in the ABS to fail or stick.

Take it back and tell him how the pedal feels but don’t tell him what to fix - he will “fix” what you say and that may not fix the problem. If you don’t trust the mechanic, find one you do. If it were me, I’d pressure bleed the entire system and see it that fixes it. I’d tackle the master cylinder if that doesn’t clear things up.

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The simple answer is that the brakes need to be bled. There are a couple of ways that air could get into the system. You have ABS and because of that, the bleed valve on each caliper has to be opened when the piston is pushed back into the caliper or it could damage the ABS pump. If the mechanic left the bleed valve open after recessing the piston, air could get in.

If you had the brake fluid flushed, and you should have, especially with ABS, it could have been done incorrectly and air got into the system that way.

If you didn’t have the brakes flushed, I’d suggest that you have that done, bleeding will be included. You will be charged for that because if you didn’t order it, you haven’t paid for it yet. It would be best if you can find a shop that uses a pressure bleeder.

Thanks guys. I will discuss options with the mechanic tomorrow, and see what he proposes. He has actually driven the car, so he knows the symptoms to.

It seems like a logical course of action is bleed then assess if there is still a problem.

Thanks Keith, for your very helpful response. To begin, you are right in your correction regarding pressure. I am describing the feeling of force on the pedal, not the actual pressure.

I understand the possibility of air in the line. I understand the possibility of the ‘straw breaking the camels back’ in the master cylinder. I think understand the ABS. Are you saying that a faulty seal or valve in the ABS means that fluid has an exit point, and thus the result is a sticky pedal? The car has 110K miles, so it is no shock that one of these things was ready to pop.

Is there a way to diagnose whether the problem is cylinder or ABS? It would be a damn shame to replace the master cylinder only to have the

I have no reason to distrust this mechanic, actually to the contrary. But I have always found that I very much appreciate having a basic understanding what is going on… As the consumer, our decisions are very stressful, and they become less so when we A) understand, and B) have a decent feeling that we are not being taken for a chump, (as tom and ray would have put it). I will not tell them what to do or replace. My questions here are simply to be as educated as I can be so that I better understand the conversations that the mechanic and I will have.

Thanks again.

Yes, I normally bleed the system and then drive the car and pound the brake pedal so ABS operates. If the pedal is still soft and but the ABS cycles on the front or rear only the master is highly likely the problem. It is also cheaper to replace the master before the ABS unit. Plus the master works ALL the time and the ABS barely ever needs to work so it is far more likely worn out.

And I disagree slightly with Keith, not all ABS units cannot withstand back-flow of brake fluid caused by compressing the pistons back into the bores. I have done that with nearly every ABS car I’ve owned since '93 from 5 different ABS suppliers and none have required that.

Shoes and pads wouldn’t be the parts involved for a front brake job on your car. It would be pads and (possibly) rotors. Shoes are only used on drum brakes, which are only used on the rear, and only on some cars, and probably not at all on your Volvo. In any event, replacing the front pads involves opening up the hydraulic system, and air can indeed enter, and usually has to be bled out after the job is done to get a firm high pedal like it was before. Either that hasn’t been done , or not done properly, or the master cylinder seals were damaged during the brake job. If so, that’s not the fault of the shop. The piston inside the master cylinder normally only moves a slight amount, but during a bleeding operation – the type where someone is pumping the pedal while a helper is opening and closing the bleeder valve – tjhat operation can move the piston to other parts cylinder which might have some debris accumulation, and tear the seals. When that happens you can get a good firm high pedal by pumping the pedal, but if you then hold your foot on the pedal it will gradually sink to the floor. Replacing the master cylinder is the remedy. It’s often possible to avoid damaging the MC seals in the first place if a pressure bleeder system is used to bleed the brakes.

While it isn’t to be expected it is not unusual for a master cylinder, wheel cylinder or caliper to fail during or soon after brake service. The hydraulic parts usually operate in a very short stroke and just ahead and behind the normal working range of the pistons deteriorated brake fluid can deposit a gummy film and even allow rust to from. In anticipation of that possibility I have for years been very aggressive in road testing brake work and on several occasions found the pedal on the floor and more work required. The OP describes the symptoms of a failing master cylinder and while it could be an ABS component hopefully it’s not. The master cylinder is a much cheaper part to replace.