Potential brake master cylinder issue

[broseph]

Failed MOT today, mechanic says when testing the brakes the first push of the pedal performed nicely but on a quick succession, release and depression, the pedal travelled far further and bit overly aggressively on all 4 corners. He suspects the brake master cylinder.

Sounds to me like the master cylinder is not replenishing itself with fluid correctly, and the extra travel is a result of having to compress a temporary gas cap caused by the quick release pulling a vacuum. If that sounds like nonsense please do call me out. Analysing fluid dynamics is my job so couldn't help theorising.

Anybody experienced a similar problem?

Unfortunately roughtrax are all out of aisin master cylinders at the moment. Anybody know anywhere else I can get them?

[NiftyNev]

Do your own tests. With engine off, pump pedal several times to deplete the vacuum. Pedal will seem hard at this stage. With foot still on pedal start engine. Pedal should drop slightly. Also drive and test if pedal position remains constant, while holding pedal in for a long period of time. If the above is ok then get a second opinion. Pumping the pedal in quick succession depletes the vacuum, which makes it seem like the brakes are not working as they should and they will not, because then you have to use more pressure to compensate for vacuum loss. Who drives a car and operates the brakes this way anyway?

Nev.

[broseph]

Do your own tests. With engine off, pump pedal several times to deplete the vacuum. Pedal will seem hard at this stage. With foot still on pedal start engine. Pedal should drop slightly. Also drive and test if pedal position remains constant, while holding pedal in for a long period of time. If the above is ok then get a second opinion. Pumping the pedal in quick succession depletes the vacuum, which makes it seem like the brakes are not working as they should and they will not, because then you have to use more pressure to compensate for vacuum loss. Who drives a car and operates the brakes this way anyway?

Nev.

I tried all of these tests. The master cylinder and booster appear to be working perfectly.
So the symptoms I can feel are as follows, there does not appear to be any binding that I can feel. What I can feel is a step in the brake when additional pressure is applied. So if I apply a medium pressure for stopping as I would driving, the brakes operate perfectly. If I apply hard pressure, there is a definite step. Just a step though, the pedal does not continue sinking. Feels to me like the additional pressure is maybe opening the lsp valve. And there's probably a little air in the rear brake set up. I changed the front brake lines and calipers just recently, didn't bleed any out the rear nipples because they were fused solid.

I think my first step will be to replace the rear brake piston thingy so I can actually bleed the brakes properly at the back.

[broseph]

Do your own tests. With engine off, pump pedal several times to deplete the vacuum. Pedal will seem hard at this stage. With foot still on pedal start engine. Pedal should drop slightly. Also drive and test if pedal position remains constant, while holding pedal in for a long period of time. If the above is ok then get a second opinion. Pumping the pedal in quick succession depletes the vacuum, which makes it seem like the brakes are not working as they should and they will not, because then you have to use more pressure to compensate for vacuum loss. Who drives a car and operates the brakes this way anyway?

Nev.

I tried all of these tests. The master cylinder and booster appear to be working perfectly.
So the symptoms I can feel are as follows, there does not appear to be any binding that I can feel. What I can feel is a step in the brake when additional pressure is applied. So if I apply a medium pressure for stopping as I would driving, the brakes operate perfectly. If I apply hard pressure, there is a definite step. Just a step though, the pedal does not continue sinking. Feels to me like the additional pressure is maybe opening the lsp valve. And there's probably a little air in the rear brake set up. I changed the front brake lines and calipers just recently, didn't bleed any out the rear nipples because they were fused solid.

I think my first step will be to replace the rear brake piston thingy so I can actually bleed the brakes properly at the back.

[Growler]

When did you last replace the brake fluid?

As old fluid can cause similar issues to what you are describing

[broseph]

When did you last replace the brake fluid?

As old fluid can cause similar issues to what you are describing

I changed the fluid just the other week however all the fluid going to the rear is still the old stuff (which was a nasty colour)

[NiftyNev]

Fluid needs a complete flush as stated. See what happens after you do this, because I believe you do not have a major problem with the master cylinder yet, as your pedal does not continue to drop. I'd suggest you get two new bleed nipples for the rear as well. The old ones may be impossible to bleed if corroded inside.

Nev.

[broseph]

Fluid needs a complete flush as stated. See what happens after you do this, because I believe you do not have a major problem with the master cylinder yet, as your pedal does not continue to drop. I'd suggest you get two new bleed nipples for the rear as well. The old ones may be impossible to bleed if corroded inside.

Nev.

Thanks, but I've gone further than just buying new bleed nipples. Bought new rear cylinders, drums, shoes, springs, flexi pipe, load valve and new lines too. Apart from the lines, it's still on its original parts which look pretty rusty. Probably overkill but I'll enjoy doing it and knowing everything is right.

Also, the load proportioning valve, the whole truck is sitting about an inch higher than standard, is this enough to warrant adjustment of the valve?