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when the engine is running, my brake pedal goes to the floor. when engine is off, I can pump up the brakes somewhat. I have changed the master cylinder and there is no fluid leak. Could the problem be with the power assist boost, or is there something else that could be the issue?
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Hi @kyvic,
What is the model year of the vehicle?
I don't know the vehicle but reading online it seems that it may have either a vacuum brake booster or a hydraulic brake booster given what booster part types are available.
If the brake pedal sinks to the floor when the engine is running then there may be a problem with the brake booster. Here’s a video that may be of some help for a vacuum brake booster. At 3:30 minutes into the video it describes why the pedal may go to the floor - faulty poppet valve in the booster.
Here’s a link that details how to test if a vacuum brake booster is working properly.
I don't know about a hydraulic brake booster but here’s the 2007-2009 GMC Yukon Service Repair Manual + Owner’s Manual. I realize that this is out of the model year range that you selected but in the Brakes > Hydraulic Brakes section, it describes the operation and repair of the hydraulic brake booster. Hopefully it may be the same for your year model.
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A brake pedal going to the floor could be indicative of having air in the system still. These can be notoriously hard to bleed all of the air out. The air will get trapped in the ABS pump and will give you a dodgy pedal. I have found activating the ABS system will help to shake the air out. There is a special bleed procedure dealers perform with their scan tool in order to activate the pump. You CAN, although I do not recommend, find a place to jam on the brakes to activate the pump and then rebleed.
If you cannot get a pedal at all, make sure you do not have another leak still, some can be harder to find as they will spray higher up or dribble along the pipe and not drip. If you got a refurbished or used M/C I would double check it to make sure its not internally leaking. They can also leak into the booster hiding a leak.
good luck
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,,, whilst searching for a solution (however not yet found) my mechanic disconnected the master cylinder from 'under the bonnet' feed and fitted 2 bolts instead to tke master cylinde, psdal was tight and didnt budge, therefore eliminating the possibility of air bubbles - nof surs if this helps.
How would u do this at home? I'm dealing with this brake issue and it's frustrating... My first time doing them on my own
@Zykearia Hyman You are doing what to which vehicle? Give more information so you can actually get more help. What are you trying to do? What make/model/year is the vehicle?
Have same problem it turned out bleeder screw snaped changed new cylinder and bleed that line was too much air traped there after that. No more spongy brake
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Air trapped in cylinder reppace cylinder then bleed that line
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My brake pedal go to the floor it stop the car but with the pedal tolow
Jose Fernandez 의
Hi I have a 2004 dodge dakota,Replaced (rear break lines rear rotors ,calipers,pads front calipers ,pads ,master cylinder, and brake booster.Bled out all air have a good hard up top pedal when truck is off .As soon as started pedal goes to the floor ? Why why ? Also the new booster check valve has three holes the old one only two.??
Tom Brunet 의
Same here mk4 bora nightmare spongy brakes im on the edge of a meltdown here 😢
Scott 의