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By Ryland
#3856146
Hi

For some time now, I've been able to pump the brake pedal to the floor when stationary.

The brakes work fine and I can get the ABS working before the pedal gives up.

Last week I bled the brakes with two mates. They said lots of air came out of the system and a couple of times when pumping the fluid through the pedal dropped to the floor with no resistance. This threw my mate off as he's never known that happen before (ex mechanic). The brakes were bled fully.

Before the bleed my brakes juddered at speed. Now they don't. Nothing else was changed.
After the bleed I feel like I've got more 'dead' travel in the pedal than before - I need to press the pedal further before anything happens.

I'm reading online that this points to a failed master cylinder? That if the peddle feels spongy and slowly depresses then the seals have gone inside. I'm not losing fluid.

For info I'm using Motul RBF600 fluid and have braided lines all around.

Do I need a new master cylinder?

Thanks
Ryland
User avatar
By mc36155
#3856257
From eric the car guy

If you have poor brake performance, you might have a problem with your master cylinder. The first step in a situation like this is to inspect the brakes at all four wheels and look for leaks. If the brakes are good and you don't see any leaks, then you might have a problem with your master cylinder.



The classic symptom of a bad master cylinder is that when you’re sitting at a stop, your brake pedal slowly sinks to the floor. You might even feel your vehicle creep forward at a stop. This usually means your master cylinder has failed. If that's the case, replace it.



Once again, you can rebuild a master cylinder, but it might be more trouble than it's worth. For that reason, I recommend you replace a master cylinder if you find it to be bad. One of the main things to remember when replacing a master cylinder is to bench-bleed it before you install it. Bench bleeding is the process of bleeding the air out of the master cylinder while it's out of the vehicle. I usually put them in a vice and work the piston after I fill the reservoir. They also make special tools that do this for you and make things easier. Sometimes a new master cylinder will even come with the parts to bench-bleed.



The takeaway, however, is that you do this before installing the master cylinder on the vehicle. If you don't, it might be very difficult to bleed the air out of the brake system after installation. Once you've installed your new master cylinder, it might be easier than you think to bleed out the system. I normally only have to bleed the system at the lines going to the master cylinder itself; I don't often have to bleed all four wheels when replacing a master cylinder.
User avatar
By 15repyt
#3856396
I wouldn't jump in and make a expensive mistake of replacing the master cylinder.

I would go round each corner and clamp off the flexi hose one at a time to try and isolate which if any corner is causing a issue. It may be a flexi hose of caliper fault.

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